[opaque pointer types] Update GetElementPtr creation APIs to
consistently accept a pointee-type argument.

Note: this also adds a new C API and soft-deprecates the old C API.

Differential Revision: https://reviews.llvm.org/D56559

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351124 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed