Rename NamedDecl::getName() to getNameAsString(). Replace a bunch of

uses of getName() with uses of getDeclName().  This upgrades a bunch of
diags to take DeclNames instead of std::strings.

This also tweaks a couple of diagnostics to be cleaner and changes
CheckInitializerTypes/PerformInitializationByConstructor to pass
around DeclarationNames instead of std::strings.

llvm-svn: 59947