Headers: use C++ inline semantics in C++ mode

When building with the 17.5.0 preview toolset for MSVC and building with
modules, the definition of _addcarry_u64 and _subborrow_u64 seem to
cause issues due to the use of GNU inline semantics. Change the headers
to prefer C++ inline semantics for C++ compilation, falling back to GNU
inlining semantics for C compilation.

This is motivated by https://github.com/microsoft/STL/issues/2520.

Differential Revision: https://reviews.llvm.org/D139749
Reviewed By: fsb4000