Revert r312224: "[ItaniumCXXABI] Always use linkonce_odr linkage for RTTI data on MinGW"

Breaks on buildbot:
http://lab.llvm.org:8011/builders/llvm-clang-x86_64-expensive-checks-win/builds/4548/steps/test-check-all/logs/stdio

The test in CodeGenCXX/virt-dtor-key.cpp tests using %itanium_abi_triple;
on non-windows platforms, this resolves to the current platform triple
(where there was no behaviour change), while on windows, it resolves to
a mingw triple (where the behaviour was intentionally changed).

llvm-svn: 312229