Catch placeholder types in DefaultLvalueConversion

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@141738 91177308-0d34-0410-b5e6-96231b3b80d8