[clang][ExtractAPI] Ensure LocationFileChecker doesn't try to traverse VFS...

[clang][ExtractAPI] Ensure LocationFileChecker doesn't try to traverse VFS when determining file path (#74071)

As part of https://reviews.llvm.org/D154130 the logic of
LocationFileChecker changed slightly to try and get the absolute
external file path instead of the name as requested when the file was
openened which would be before VFS mappings in our usage. Ensure that we
only check against the name as requested instead of trying to generate
the external canonical file path.

rdar://115195433