[clangd] Fix isSpelledInSource crash on invalid FileID (#76668)

This fixes the crash reported in #76667 and adds an initial unit test
for isSpelledInSource().

Note that in that issue there was still some underlying corrupted AST,
but this at least makes isSpelledInSource() robust to it.

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Co-authored-by: Younan Zhang <zyn7109@gmail.com>