[COFF] Handle .eh_frame$symbol as associative comdat for MinGW

This matches how it is done for .xdata and .pdata already.

On i386, the symbol name in the section name suffix does not contain
the extra underscore prefix.

This is one part of a fix for PR42217.

Differential Revision: https://reviews.llvm.org/D63350

llvm-svn: 363456