[MinGW] Ignore the --enable-auto-import flag

In GNU ld, this option is enabled by default, but can be set
to reduce some warnings.

For lld, ignore the flag (for now); in case linking still succeeds
everything should be fine, if not, it should be clear to the user
what part failed (possibly requiring adjusting the user project
to not rely on this feature), instead of straight out failing due to
an unknown flag.

Differential Revision: https://reviews.llvm.org/D39330

llvm-svn: 316693