[frontend] Don't include the C++ stdlib for -x assembler-with-cpp

The new C++ stdlib warning added in r335081 gets
triggered when compiling an assembly file with -x assembler-with-cpp.
This commit ensures that the C++ stdlib is not included when compiling assembly.
In general, it's not really useful to include the C++ stdlib search path when
compiling assembly source.

rdar://41359632

Differential Revision: https://reviews.llvm.org/D48736

llvm-svn: 335940