[clang][NFC] DeclPrinter: use NamedDecl::getDeclName instead of...

[clang][NFC] DeclPrinter: use NamedDecl::getDeclName instead of NamedDecl::printName to print the name of enumerations, namespaces and template parameters.

NamedDecl::printName will print the pretty-printed name of the entity, which
is not what we want here (we should print "enum { e };" instead of "enum
(unnamed enum at input.cc:1:5) { e };").

For now only DecompositionDecl and MDGuidDecl have an overloaded printName so
this does not result in any functional change, but this change is needed since
I will be adding overloads to better handle unnamed entities in diagnostics.