[opaque pointer types] Make EmitCall pass Function Types to

CreateCall/Invoke.

Also, remove the getFunctionType() function from CGCallee, since it
accesses the pointee type of the value. The only use was in EmitCall,
so just inline it into the debug assertion.

This is the last of the changes for Call and Invoke in clang.

Differential Revision: https://reviews.llvm.org/D57804

llvm-svn: 353356