VS: Update support for LLVM-vs* toolsets from LLVM 5.0
Revert !131 (merged). Since at least LLVM 5.0 the VS integration of the LLVM toolchain now mimics cl and accepts MSVC-style command-line arguments (unlike Microsoft Clang/C2).
Fixes: #17193 (closed), #17235 (closed)
Topic-rename: vs-clang-llvm-support
Edited by Brad King