[COFF] Add -exclude-all-symbols for MinGW

This is a private undocumented option, intended to be used by
the MinGW driver frontend.

Also restructure the condition to put if (Config->MinGW) first.
This changes the behaviour for the tautological combination of
-export-all-symbols without -lldmingw.

Differential Revision: https://reviews.llvm.org/D58380

llvm-svn: 354386