2. Added partial debug support for mingw\cygwin targets (the same as
   Linux\ELF). Please note, that currently mingw\cygwin uses 'stabs' format
   for storing debug info by default, thus many (runtime) libraries has
   this information included. These formats shouldn't be mixed in one binary
   ('stabs' & 'DWARF'), otherwise binutils tools will be confused.

llvm-svn: 31311