The FP constant evaluator was missing a few cases of unary operators that return...

The FP constant evaluator was missing a few cases of unary operators that return floats
but whose operand isn't a float:  specifically, __real__ and __imag__.  Instead
of filtering these out, just implement them.

Fixes <rdar://problem/7958272>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@103307 91177308-0d34-0410-b5e6-96231b3b80d8