Update the MSVC Compatibility document

It hasn't been updated to reflect the progress we've made.  We've fuzz
tested VFTables, VBTables, layout, and RTTI data.  We support lambdas
that are compatible with their scheme.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212193 91177308-0d34-0410-b5e6-96231b3b80d8