Member pointers refer to data or function members of a `CXXRecordDecl`,
which require a `MSInheritanceAttr` in order to be complete. Without that
we cannot calculate the size of a member pointer in memory. The attempt
has been causing a crash further down in the clang AST context. In order
to implement the feature, DWARF will need a new attribtue to convey the
information. For the moment, this patch teaches LLDB to handle to
situation and avoid the crash.
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