MSVC: Use -external:I flag without space to support Clang tools
Since !4766 (merged) we use the new -external:I
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 (closed)
Backport: release