ubsan: Implement memory permission validation for vtables.

If the pointer passed to the getVtablePrefix function was read from a freed
object, we may end up following pointers into objects on the heap and
printing bogus dynamic type names in diagnostics. However, we know that
vtable pointers will generally only point into memory mapped from object
files, not objects on the heap.

This change causes us to only follow pointers in a vtable if the vtable
and one of the virtual functions it points to appear to have appropriate
permissions (i.e. non-writable, and maybe executable), which will generally
exclude heap pointers.

Only enabled for Linux; this hasn't been tested on FreeBSD, and vtables are
writable on Mac (PR24782) so this won't work there.

Differential Revision: http://reviews.llvm.org/D12790

llvm-svn: 247484