[libc++] Fix inline attribute for non-MSVC Windows

gcc and clang in gcc compatibility mode do not accept __forceinline. Use
the gcc attribute for them instead.

Differential Revision: https://reviews.llvm.org/D24678

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@281766 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed