[clang] [Clang][Frontend] Fix a crash when -Wdocumentation is used (PR #68525)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > Is there a way we could come up with a test for this?
> 
> Unfortunately, I don't think so. I cannot reduce the 600k lines of preprocessed code to a small test case that will crash the clang frontend.

Have you tried using creduce or other such tool? (We generally don't accept patches [without test coverage](https://llvm.org/docs/DeveloperPolicy.html#test-cases) unless it really isn't possible to test the changes, that doesn't appear to be the situation here though.)

https://github.com/llvm/llvm-project/pull/68525