[FileCollector] Fix that the file system case-sensitivity check was inverted

real_path returns an `std::error_code` which evaluates to `true` in case an
error happens and `false` if not. This code was checking the inverse, so
case-insensitive file systems ended up being detected as case sensitive.

Tested using an LLDB reproducer test as we anyway need a real file system and
also some matching logic to detect whether the respective file system is
case-sensitive (which the test is doing via some Python checks that we can't
really emulate with the usual FileCheck logic).

Fixes rdar://67003004

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D96795