Fix char_traits functions for GCC compatibility.

GCC 4.9 fails to inline these functions at -O1 because they are used
indirectly. Declare them as inline instead of always_inline. Discussion
in GCC bugreport: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63220


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@217961 91177308-0d34-0410-b5e6-96231b3b80d8