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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237728 91177308-0d34-0410-b5e6-96231b3b80d8