[clang] [Clang] Introduce __builtin_is_modifiable_lvalue() (PR #132524)

[Andrew Pinski via cfe-commits]

pinskia wrote:

FYI I filed https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123527 to keep track it on the GCC side of things. Looks like there has not been any movement on this since March of last year either.

https://github.com/llvm/llvm-project/pull/132524