[clang-tidy] Add a Standalone diagnostics mode to clang-tidy

Adds a flag to `ClangTidyContext` that is used to indicate to checks that fixes will only be applied one at a time.
This is to indicate to checks that each fix emitted should not depend on any other fixes emitted across the translation unit.
I've currently implemented the `IncludeInserter`, `LoopConvertCheck` and `PreferMemberInitializerCheck` to use these support these modes.

Reasoning behind this is in use cases like `clangd` it's only possible to apply one fix at a time.
For include inserter checks, the include is only added once for the first diagnostic that requires it, this will result in subsequent fixes not having the included needed.

A similar issue is seen in the `PreferMemberInitializerCheck` where the `:` will only be added for the first member that needs fixing.

Fixes emitted in `StandaloneDiagsMode` will likely result in malformed code if they are applied all together, conversely fixes currently emitted may result in malformed code if they are applied one at a time.
For this reason invoking `clang-tidy` from the binary will always with `StandaloneDiagsMode` disabled, However using it as a library its possible to select the mode you wish to use, `clangd` always selects `StandaloneDiagsMode`.

This is an example of the current behaviour failing
```lang=c++
struct Foo {
  int A, B;
  Foo(int D, int E) {
    A = D;
    B = E; // Fix Here
  }
};
```
Incorrectly transformed to:
```lang=c++
struct Foo {
  int A, B;
  Foo(int D, int E), B(E) {
    A = D;
     // Fix Here
  }
};
```
In `StandaloneDiagsMode`, it gets transformed to:
```lang=c++
struct Foo {
  int A, B;
  Foo(int D, int E) : B(E) {
    A = D;
     // Fix Here
  }
};
```

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D97121

1 files changed, 2 insertions, 1 deletions

diff --git a/clang-tools-extra/clang-tidy/performance/UnnecessaryValueParamCheck.cpp b/clang-tools-extra/clang-tidy/performance/UnnecessaryValueParamCheck.cpp
index 2a9f9b7..ba2b89c 100644
--- a/clang-tools-extra/clang-tidy/performance/UnnecessaryValueParamCheck.cpp
+++ b/clang-tools-extra/clang-tidy/performance/UnnecessaryValueParamCheck.cpp
@@ -69,7 +69,8 @@ UnnecessaryValueParamCheck::UnnecessaryValueParamCheck(
     StringRef Name, ClangTidyContext *Context)
     : ClangTidyCheck(Name, Context),
       Inserter(Options.getLocalOrGlobal("IncludeStyle",
-                                        utils::IncludeSorter::IS_LLVM)),
+                                        utils::IncludeSorter::IS_LLVM),
+               areDiagsSelfContained()),
       AllowedTypes(
           utils::options::parseStringList(Options.get("AllowedTypes", ""))) {}