[MS ABI] Allow a member pointers' converted type to change

Member pointers in the MS ABI are tricky for a variety of reasons.
The size of a member pointer is indeterminate until the program reaches
a point where the representation is required to be known.  However,
*pointers* to member pointers may exist without knowing the pointee
type's representation.  In these cases, we synthesize an opaque LLVM
type for the pointee type.

However, we can be in a situation where the underlying member pointer's
representation became known mid-way through the program.  To account for
this, we attempted to manicure CodeGen's type-cache so that we can
replace the opaque member pointer type with the real deal while leaving
the pointer types unperturbed.  This, unfortunately, is a problematic
approach to take as we will violate CodeGen's invariants.

These violations are mostly harmless but let's do the right thing
instead: invalidate the type-cache if a member pointer's LLVM
representation changes.

This fixes PR26313.

llvm-svn: 258839