[clang] Itanium mangler constructors
The Itanium mangler constructors use both NSDMI and explicit member
construction for default values. This is confusing.
*) Use NSDMIs wherever possible
*) Use forwarding ctor for the nesting case with an
llvm::raw_null_ostream (and explicitly set NullOut flag in that ctor).
*) Copy the ModuleSubstitutions. This is a bug with no effect in the
current mangling, but not in the newer mangling.
Reviewed By: ChuanqiXu
Differential Revision: https://reviews.llvm.org/D119550