MSVC: Use -external:I flag without space to support Clang tools

Since commit f29e1874 (Compiler/MSVC: use the `-external:I` flag for
system includes, 2020-05-19, v3.22.0-rc1~593^2) we use the new flag
followed by the include directory as a separate argument.  Some versions
of `clang-cl` and `clang-tidy` do not support the flag unless the
include directory is attached to it, so use that form instead.

Fixes: #22979
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