VS: Update support for LLVM-vs* toolsets from LLVM 5.0

Revert commit v3.7.0-rc1~25^2 (VS: Recognize VS/LLVM toolset names as
Clang, 2016-09-28).  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, #17235
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