As discussed in b4f6b26b "SAL_DLLPUBLIC_RTTI for
proper RTTI visibility for LLVM," RTTI-based -fsanitize= checks with Clang on
Linux need special precautions to make RTTI symbols visible across DSOs.  The
approach taken there, as well as in 598d8194
"Visible function type RTTI for Clang -fsanitize=function," was to add explicit
SAL_DLLPUBLIC_RTTI annontations to relevant type definitions.  However, for
-fsanitize=vptr that would have required many more of those, so it appears
easier to "misuse" -fsanitize-ms-compat in that case, which happens to give all
RTTI symbols default visibility (while otherwise still honoring our
SAL_DLLPUBLIC/PRIVATE annotations).

The SAL_DLLPUBLIC_RTTI annotations from 598d8194
"Visible function type RTTI for Clang -fsanitize=function" can likely be removed
again.

Change-Id: Ibeff7ab8c908111a7dc66ff0677204f112b24db8