[clang] [clang] Allow C-style casts in constexpr in MS compatible mode (PR #197005)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> OK, so the macro has been copied to some other file, and was not updated at the same time as the fix to winnt.h. So, how about the next Q: "If so, can that other file just be updated correspondingly, with the same change?"
> 
> This just seems like a whole lot of workaround cruft, for something that's trivial to fix in the macro definition -- and, furthermore, already HAS been fixed, years ago, in most headers on most platforms -- including the more-commonly-used windows headers.

Another way to think about it is: this blesses an *extremely* common code pattern in the wild that has existed for 40+ years: https://sourcegraph.com/search?q=context:global+lang:C%2B%2B+%5C*%5C%290%5C%29%5C-%5C%3E+count:1000000&patternType=regexp&sm=0

I'm sure some of those 610k+ results are false positives, hits from C code, not used in constant expressions, etc. but this still seems like a very reasonable change IMO.

https://github.com/llvm/llvm-project/pull/197005