[CMake][WinMsvc] Replace MSVC_BASE/WINSDK_BASE with LLVM_WINSYSROOT

- Using LLVM_WINSYSROOT would pick up DIA SDK path automatically,
  otherwise llvm-pdbutil has no DIA support.
- Add MSVC_VER to specify VC tools version.
- Make MSVC_VER/WINSDK_VER optional. If not specified, use the highest
  version number like the driver does.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D117852