[llvm-c] Add functions for enabling and creating opaque pointers

This is based on https://reviews.llvm.org/D125168 which adds a
wrapper to allow use of opaque pointers from the C API.

I added an opaque pointer mode test to echo.ll, and to fix assertions
that forbid the use of mixed typed and opaque pointers that were
triggering in it I had to also add wrappers for setOpaquePointers()
and isOpaquePointer().

I also changed echo.ll to remove a bitcast i32* %x to i8*, because
passing it through llvm-as and llvm-dis was generating a
%0 = bitcast ptr %x to ptr, but when building that same bitcast in
echo.cpp it was getting elided by IRBuilderBase::CreateCast
(https://github.com/llvm/llvm-project/blob/08ac66124874d70dab63c731da0244f9e29ef168/llvm/include/llvm/IR/IRBuilder.h#L1998-L1999).

Differential Revision: https://reviews.llvm.org/D125183