Create specialization of -Wgnu-statement-expression for expressions found in macros.

-Wgnu-statement-expression currently warns for both direct source uses of statement expressions but also macro expansions; since they may be used by macros to avoid multiple evaluation of macro arguments, engineers might want to suppress warnings when statement expressions are expanded from macros but see them if introduced directly in source code.

Differential Revision: https://reviews.llvm.org/D126522