[MCA] Fix MSVC 19.16 build with libc++

MSVC (19.16) wants to see the definition of Instruction in
`std::pair<unsigned, const Instruction &> SourceRef` to decide
if it is assignable.

Patch by Orivej Desh.

Differential Revision: https://reviews.llvm.org/D65844

llvm-svn: 368436