[solaris] Convert the support library to C++ to fix -std=c++11 build

Convert the Solaris xlocale.c compatibility library from plain C to C++
in order to fix the build failures caused by the addition of -std=c++11
to LIBCXX_COMPILE_FLAGS. The additional flag got propagated to the C
file, resulting in error with strict compilers.

Differential Revision: https://reviews.llvm.org/D25431

llvm-svn: 284494