[ms-cxxabi] Implement MSVC virtual base adjustment

While we can't yet emit vbtables, this allows us to find virtual bases
of objects constructed in other TUs.

This make iostream hello world work, since basic_ostream virtually
inherits from basic_ios.

Differential Revision: http://llvm-reviews.chandlerc.com/D795

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182870 91177308-0d34-0410-b5e6-96231b3b80d8