Revert "[LLD] [COFF] Don't try to detect MSVC installations in mingw mode"

This reverts commit 389bfbd6.

This commit broke the compiler-rt/test/profile/Windows/coverage-weak-lld.cpp
testcase. This testcase builds and links in an MSVC environment,
but explicitly passes -lldmingw to the linker, to opt in to
certain mingw-style behaviours regarding weak symbols, since
effb87df. This patch broke that
test, which now errored out with these errors:

    lld-link: error: could not open 'libuuid.a': no such file or directory
    lld-link: error: could not open 'libLIBCMT.a': no such file or directory
    lld-link: error: could not open 'libOLDNAMES.a': no such file or directory