Consider removing the search path fixup in bindgen

As I mentioned in #330 (comment), I think we should remove the search path fixup.

The reasons are:

• There are many arguments which can affect search path selection. In addition to --target (which the fixup detects and disables itself with), -stdlib, -mmacos-version-min, etc. can also affect search path selection. It means if someone forgets to disable the fixup via --target when they use those arguments, they may have multiple search path set for stdlib, which can lead to unpredictable issues.
• This fixup effectively hides real issues that people need to specify arguments for having the correct search path.

I don't see any evidence indicating that libclang cannot do the same search path selection as command line clang. @emilio pointed out that #242 is an issue because of lack of this fixup, but I don't think it's convincing since the reporter doesn't provide the arguments they use, so it is unclear what really should happen.

Start by locating the search path fixup referenced in this issue and read the discussion in #330. Review #242 and compare the behavior with the command-line clang search-path arguments mentioned here; done means the fixup is removed without masking required arguments and relevant behavior is covered by tests.