[clang] [clang][HIP][CUDA] Disambiguate lambdas from Microsoft attributes (PR #214247)

[Reid Kleckner via cfe-commits]

rnk wrote:

A used codex to analyze the PR ([raw transcript](https://gist.github.com/rnk/55faeb309e6696433d3e112fff222528), and it suggested an alternative pathway that involves editing *just* the *tenative parsing* codepaths.

Specifically, if we only need to do this disambiguation for vexing-parse cases, where we need to distinguish between parameters which could either be an expression or a parameter declaration, you should be able to limit the changes to these code locations:

[  clang/lib/Parse/ParseTentative.cpp:1736](https://github.com/llvm/llvm-project/blob/5188dfc273470d3c47eac980128f66e7fb898b3d/clang/lib/Parse/ParseTentative.cpp#L1736)
```
  ParsedAttributes attrs(AttrFactory);
  MaybeParseMicrosoftAttributes(attrs);

  // decl-specifier-seq
  TPResult TPR = isCXXDeclarationSpecifier(...);
```

The value of narrowing the scope of the change would be to avoid the compile time hit of tentatively parsing forwards every time we see a left square bracket in a declaration sequence, something which might be a microsoft attribute. However, this could happen approximately never, in which case, maybe you're fix is better. I think lambdas often appear in type-or-expression contexts, so my sense is that it happens fairly often, and if we can limit the tentative parse to just parameter lists, that seems like it would be a win.

https://github.com/llvm/llvm-project/pull/214247