Implement the missing pieces of Evaluate for _Complex types. With that complete...

Implement the missing pieces of Evaluate for _Complex types.  With that complete, remove some code from CGExprConstant which is no longer necessary.  While I'm here, a couple minor tweaks to _Complex-in-C++. (Specifically, make _Complex types literal types, and don't warn for _Complex int.)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@147840 91177308-0d34-0410-b5e6-96231b3b80d8