SymbolFile: ensure that we have a value before invoking `getBitWidth`

Ensure that the variant returned by `member->getValue()` has a value and
is not `Empty`.  Failure to do so will trigger an assertion failure in
`llvm::pdb::Variant::getBitWidth()`.  This can occur when the `static`
member is a forward declaration.

Differential Revision: https://reviews.llvm.org/D146536
Reviewed By: sgraenitz