Introduce _BitInt, deprecate _ExtInt

WG14 adopted the _ExtInt feature from Clang for C23, but renamed the
type to be _BitInt. This patch does the vast majority of the work to
rename _ExtInt to _BitInt, which accounts for most of its size. The new
type is exposed in older C modes and all C++ modes as a conforming
extension. However, there are functional changes worth calling out:

* Deprecates _ExtInt with a fix-it to help users migrate to _BitInt.
* Updates the mangling for the type.
* Updates the documentation and adds a release note to warn users what
is going on.
* Adds new diagnostics for use of _BitInt to call out when it's used as
a Clang extension or as a pre-C23 compatibility concern.
* Adds new tests for the new diagnostic behaviors.

I want to call out the ABI break specifically. We do not believe that
this break will cause a significant imposition for early adopters of
the feature, and so this is being done as a full break. If it turns out
there are critical uses where recompilation is not an option for some
reason, we can consider using ABI tags to ease the transition.

1 files changed, 2 insertions, 2 deletions

diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp b/clang/lib/CodeGen/CodeGenFunction.cpp
index d87cf2d..ed43adf 100644
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/clang/lib/CodeGen/CodeGenFunction.cpp
@@ -243,7 +243,7 @@ TypeEvaluationKind CodeGenFunction::getEvaluationKind(QualType type) {
     case Type::Enum:
     case Type::ObjCObjectPointer:
     case Type::Pipe:
-    case Type::ExtInt:
+    case Type::BitInt:
       return TEK_Scalar;
 
     // Complexes.
@@ -2207,7 +2207,7 @@ void CodeGenFunction::EmitVariablyModifiedType(QualType type) {
     case Type::ObjCObject:
     case Type::ObjCInterface:
     case Type::ObjCObjectPointer:
-    case Type::ExtInt:
+    case Type::BitInt:
       llvm_unreachable("type class is never variably-modified!");
 
     case Type::Adjusted: