[opaque pointer type] Provide the ability to pass an explicit type when...

[opaque pointer type] Provide the ability to pass an explicit type when creating a CallInst through IRBuilder

A use for this will be added to Clang shortly. I haven't hit the
specific cases I want to cleanup in LLVM just yet.

A utility that avoids the need for this in direct calls to
llvm::Functions will be added too - since in that case the type can be
retrieved from the llvm::Function directly.

llvm-svn: 237728