Emit the C++ dialect in -gmodules .pcm files.

Because of commit: https://reviews.llvm.org/D104291 the -gmodules .pcm
files do not have the same DW_AT_language dialect as the .o file. This
was a simple matter of passing the DebugStrictDwarf flag to the
PCHContainerGenerator object's CodeGenOpts from the CompilerInstance
passed in to it.

Before this change if you ran dwarfdump on the gmodule cache folder
you would get DW_AT_language (DW_LANG_C_plus_plus) even when using
-std=c++14 with clang

Patch by Shubham Rastogi!

Differential Revision: https://reviews.llvm.org/D116790