[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