Consider removing the search path fixup in bindgen

[upsuper]

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.

[emilio]

It's a shame that the old issues in Yamakaky/rust-bindgen are no longer there. I believe there were some actual legitimate issues, but oh well.

As I said I think this should be optional and opt-out, but I guess we can also make it opt-in, doing a breaking version bump, see if it breaks for a lot of people, and roll it back if it does.