[clang-tools-extra] [clangd] Fix SIGSEGV crash when receiving a textDocument/didOpen request with the URI pointing to a directory (PR #177834)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

> I think its probably the FileEntryRef object (`Val` in frame 0) that has a null internal pointer which is getting dereferenced

Yeah, that seems plausible as a downstream consequence of not satisfying the preconditions of `getVirtualFileRef()`.

It does mean that a few stack frames got inlined and didn't show up in the segfault's backtrace. I do prefer `Debug` builds for investigating crashes like this, rather than `RelWithDebugInfo` builds, because their backtraces are more complete (and also because assertions are enabled in them, which alert us to problems closer to their source).

https://github.com/llvm/llvm-project/pull/177834