Reland 192220 "Abstract out parts of thunk emission code, add support for simple thunks when using -cxx-abi microsoft" with relaxed assertions

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192285 91177308-0d34-0410-b5e6-96231b3b80d8
clang/test/Coverage/codegen-next.m: Exclude this for targeting x86_64-win32.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192286 91177308-0d34-0410-b5e6-96231b3b80d8