[clang][ExtractAPI] fix a couple crashes when used via libclang (#132297)

This PR fixes two crashes in ExtractAPI that occur when decls are
requested via libclang:

- A null-dereference would sometimes happen in
`DeclarationFragmentsBuilder::getFragmentsForClassTemplateSpecialization`
when the template being processed was loaded indirectly via a typedef,
with parameters filled in. The first commit loads the template parameter
locations ahead of time to perform a null check before dereferencing.
- An assertion (or another null-dereference) was happening in
`CXXRecordDecl::bases` when processing a forward-declaration (i.e. a
record without a definition). The second commit guards the use of
`bases` in `ExtractAPIVisitorBase::getBases` by first checking that the
decl in question has a complete definition.

The added test `extract-api-cursor-cpp` adds tests for these two
scenarios to protect against the crash in the future.

Fixes rdar://140592475, fixes rdar://123430367

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