Support __PRETTY_FUNCTION__ and friends in Obj-C methods.

Add CodeGenFunction::EmitUnsupportedLValue 
 - Gives error and returns undef value.

Swap some asserts() over to using EmitUnsupportedLValue
 - Rumor has it users (and even some developers) prefer carat
   diagnostics to backtraces.
 - Works better in Release-Asserts to boot.

llvm-svn: 55328