Revert "Move dllimport name mangling to IR mangler."

Revert "Move dllimport name mangling to IR mangler."

This reverts commit r239437.

This broke clang-cl self-hosts. We'd end up calling the __imp_ symbol
directly instead of using it to do an indirect function call.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239502 91177308-0d34-0410-b5e6-96231b3b80d8