Refactor computation of whether a variable declaration's type should be merged
with a prior declaration. No functionality change intended.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189850 91177308-0d34-0410-b5e6-96231b3b80d8
Switched FormatAttr to using an IdentifierArgument instead of a StringArgument since that is a more accurate modeling.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189851 91177308-0d34-0410-b5e6-96231b3b80d8