[CMake] Allow overriding MSVC_DIA_SDK_DIR via CMake

This eases using it in cross-msvc setups.

Differential Revision: https://reviews.llvm.org/D69239

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375485 91177308-0d34-0410-b5e6-96231b3b80d8