[clang][ExprConst] Add diagnostics for invalid binary arithmetic (#118475)

... between unrelated declarations or literals.

Leaving this small (I haven't run the whole test suite locally) to get
some feedback on the wording and implementation first.

The output of the sample in
https://github.com/llvm/llvm-project/issues/117409 is now:
```console
./array.cpp:57:6: warning: expression result unused [-Wunused-value]
   57 |   am - aj.af();
      |   ~~ ^ ~~~~~~~
./array.cpp:70:8: error: call to consteval function 'L::L<bx>' is not a constant expression
   70 |   q(0, [] {
      |        ^
./array.cpp:57:6: note: arithmetic on addresses of literals has unspecified value
   57 |   am - aj.af();
      |      ^
./array.cpp:62:5: note: in call to 'al(&""[0], {&""[0]})'
   62 |     al(bp.af(), k);
      |     ^~~~~~~~~~~~~~
./array.cpp:70:8: note: in call to 'L<bx>({})'
   70 |   q(0, [] {
      |        ^~~~
   71 |     struct bx {
      |     ~~~~~~~~~~~
   72 |       constexpr operator ab<g<l<decltype(""[0])>::e>::e>() { return t(""); }
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   73 |     };
      |     ~~
   74 |     return bx();
      |     ~~~~~~~~~~~~
   75 |   }());
      |   ~~~
```

The output for 
```c++
int a, b;
constexpr int n = &b - &a
```

is now:
```console
./array.cpp:80:15: error: constexpr variable 'n' must be initialized by a constant expression
   80 | constexpr int n = &b - &a;
      |               ^   ~~~~~~~
./array.cpp:80:22: note: arithmetic involving '&b' and '&a' has unspecified value
   80 | constexpr int n = &b - &a;
      |                      ^
1 error generated.

```

4 files changed, 6 insertions, 6 deletions

diff --git a/clang/test/AST/ByteCode/builtin-functions.cpp b/clang/test/AST/ByteCode/builtin-functions.cpp
index 7237640..7034e8c 100644
--- a/clang/test/AST/ByteCode/builtin-functions.cpp
+++ b/clang/test/AST/ByteCode/builtin-functions.cpp
@@ -1010,7 +1010,7 @@ namespace FunctionStart {
   void a(void) {}
   static_assert(__builtin_function_start(a) == a, ""); // both-error {{not an integral constant expression}} \
                                                        // ref-note {{comparison against opaque constant address '&__builtin_function_start(a)'}} \
-                                                       // expected-note {{comparison of addresses of literals has unspecified value}}
+                                                       // expected-note {{comparison of addresses of potentially overlapping literals has unspecified value}}
 }
 
 namespace BuiltinInImplicitCtor {
diff --git a/clang/test/AST/ByteCode/cxx20.cpp b/clang/test/AST/ByteCode/cxx20.cpp
index dea4055..268362c 100644
--- a/clang/test/AST/ByteCode/cxx20.cpp
+++ b/clang/test/AST/ByteCode/cxx20.cpp
@@ -115,7 +115,7 @@ constexpr auto name1() { return "name1"; }
 constexpr auto name2() { return "name2"; }
 
 constexpr auto b3 = name1() == name1(); // ref-error {{must be initialized by a constant expression}} \
-                                        // ref-note {{comparison of addresses of literals}}
+                                        // ref-note {{comparison of addresses of potentially overlapping literals}}
 constexpr auto b4 = name1() == name2();
 static_assert(!b4);
 
diff --git a/clang/test/AST/ByteCode/functions.cpp b/clang/test/AST/ByteCode/functions.cpp
index 10bea3a..66693a1 100644
--- a/clang/test/AST/ByteCode/functions.cpp
+++ b/clang/test/AST/ByteCode/functions.cpp
@@ -208,11 +208,11 @@ namespace Comparison {
 
   constexpr bool u13 = pf < pg; // both-warning {{ordered comparison of function pointers}} \
                                 // both-error {{must be initialized by a constant expression}} \
-                                // both-note {{comparison between '&f' and '&g' has unspecified value}}
+                                // both-note {{comparison between pointers to unrelated objects '&f' and '&g' has unspecified value}}
 
   constexpr bool u14 = pf < (void(*)())nullptr; // both-warning {{ordered comparison of function pointers}} \
                                                 // both-error {{must be initialized by a constant expression}} \
-                                                // both-note {{comparison between '&f' and 'nullptr' has unspecified value}}
+                                                // both-note {{comparison between pointers to unrelated objects '&f' and 'nullptr' has unspecified value}}
 
 
 
diff --git a/clang/test/AST/ByteCode/literals.cpp b/clang/test/AST/ByteCode/literals.cpp
index 662823c..3d415a9 100644
--- a/clang/test/AST/ByteCode/literals.cpp
+++ b/clang/test/AST/ByteCode/literals.cpp
@@ -194,7 +194,7 @@ namespace PointerComparison {
   constexpr void *qv = (void*)&s.b;
   constexpr bool v1 = null < (int*)0;
   constexpr bool v2 = null < pv; // both-error {{must be initialized by a constant expression}} \
-                                 // both-note {{comparison between 'nullptr' and '&s.a' has unspecified value}}
+                                 // both-note {{comparison between pointers to unrelated objects 'nullptr' and '&s.a' has unspecified value}}
 
   constexpr bool v3 = null == pv; // ok
   constexpr bool v4 = qv == pv; // ok
@@ -202,7 +202,7 @@ namespace PointerComparison {
   constexpr bool v5 = qv >= pv;
   constexpr bool v8 = qv > (void*)&s.a;
   constexpr bool v6 = qv > null; // both-error {{must be initialized by a constant expression}} \
-                                 // both-note {{comparison between '&s.b' and 'nullptr' has unspecified value}}
+                                 // both-note {{comparison between pointers to unrelated objects '&s.b' and 'nullptr' has unspecified value}}
 
   constexpr bool v7 = qv <= (void*)&s.b; // ok