Simplify/clean up debug info suppression in CodeGenFunction

Simplify/clean up debug info suppression in CodeGenFunction

CodeGenFunction is run on only one function - a new object is made for
each new function. I would add an assertion/flag to this effect, but
there's an exception: ObjC properties involve emitting helper functions
that are all emitted by the same CodeGenFunction object, so such a check
is not possible/correct.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189277 91177308-0d34-0410-b5e6-96231b3b80d8