Catch placeholder types in DefaultLvalueConversion

and DefaultFunctionArrayLvalueConversion.  To prevent
significant regression for should-this-be-a-call fixits,
and to repair some such regression from the introduction of
bound member placeholders, make those placeholder checks
try to build calls appropriately.  Harden the build-a-call
logic while we're at it.

llvm-svn: 141738