[ClangASTContext] Extract VTable pointers from C++ objects

This patch processes the case of retrieving a virtual base when the object is
already read from the debuggee memory.

To achieve that ValueObject::GetCPPVTableAddress was removed and was
reimplemented in ClangASTContext (because access to the process is needed to
retrieve the VTable pointer in general, and because this is the only place that
used old version of ValueObject::GetCPPVTableAddress).

This patch allows to use real object's VTable instead of searching virtual bases
by offsets restored by MicrosoftRecordLayoutBuilder. PDB has no enough info to
restore VBase offsets properly, so we have to read real VTable instead.

Differential revision: https://reviews.llvm.org/D53506

llvm-svn: 346669