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

[Andrew Pinski via cfe-commits]

pinskia wrote:

Note the GCC implementtion of __builtin_is_modifiable_lvalue that I have implemented (unlike __builtin_constant_p which in some but not all cases) throws away the full expression that is inside __builtin_is_modifiable_lvalue since it is fully implemented in the parser rather than a mix of the middle-end and parser. In a similar way __builtin_choose_expr is handled for the non chosen side. 

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