Convert IdentifierInfo's to be printed the same as DeclarationNames

Convert IdentifierInfo's to be printed the same as DeclarationNames
with implicit quotes around them.  This has a bunch of follow-on
effects and requires tweaking to a whole lot of code.  This causes
a regression in two tests (xfailed) by causing it to emit things like:

  Line 10: duplicate interface declaration for category 'MyClass1' ('Category1')

instead of:

  Line 10: duplicate interface declaration for category 'MyClass1(Category1)'

I will fix this in a follow-up commit.

As part of this, I had to start switching stuff to use ->getDeclName() instead
of Decl::getName() for consistency.  This is good, but I was planning to do this
as an independent patch.  There will be several follow-on patches
to clean up some of the mess, but this patch is already too big.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@59917 91177308-0d34-0410-b5e6-96231b3b80d8