[Sanitizers] [MinGW] Produce undecorated symbols for /export: directives when in MinGW mode

In MinGW mode, the /export: directives in object files are interpreted
differently; the symbols are not assumed to be decorated.

Since the GNU linker doesn't support the /alternatename and /include
directives, there's no such distinction for them in lld. This assumes
that the resulting sanitizer libraries will be linked by lld.

Differential Revision: https://reviews.llvm.org/D51882

llvm-svn: 343015