AST: Make getEffectiveDeclContext() a member function of ItaniumMangleContextImpl. NFCI.

In an upcoming change we are going to need to access mangler state
from the getEffectiveDeclContext() function. Therefore, make it a
member function of ItaniumMangleContextImpl. Any callers that are
not currently members of ItaniumMangleContextImpl or CXXNameMangler
are made members of one or the other depending on where they are
called from.

Differential Revision: https://reviews.llvm.org/D116773