[COFF] Treat .xdata/.pdata$<sym> as implicitly associative to <sym> for MinGW

MinGW configurations don't use associative comdats, as GNU ld doesn't
support that. Instead they produce normal comdats named .text$sym,
.xdata$sym and .pdata$sym.

GNU ld doesn't discard any comdats starting with .xdata or .pdata,
even if --gc-sections is used (while it does discard other unreferenced
comdats), regardless of what symbol name is used after the $ separator.

For LLD, treat any such comdat as implicitly associative to the base
symbol. This requires maintaining a map from symbol name to section
number, but that is only maintained when the MinGW flag has been
enabled.

Differential Revision: https://reviews.llvm.org/D49700

llvm-svn: 339058