[clang][nearly-NFC] Remove some superfluous uses of NamedDecl::getNameAsString

[clang][nearly-NFC] Remove some superfluous uses of NamedDecl::getNameAsString

`OS << ND->getDeclName();` is equivalent to `OS << ND->getNameAsString();`
without the extra temporary string.

This is not quite a NFC since two uses of `getNameAsString` in a
diagnostic are replaced, which results in the named entity being
quoted with additional "'"s (ie: 'var' instead of var).