[Clang] Prioritise built-in headers, even on musl.

[al45tair]

Clang was putting its built-in headers at the end of the search path if running on musl; this was a mistake, because it breaks libc++, as the latter tries to include the built-in header and then the #include_next in the built-in header fails.

The right solution here is to have the built-in headers remain in their usual location in the search path, and then if it's desirable to override them for musl, have them explicitly include the musl header with #include_next. This is the solution that is already in use for other platforms.

rdar://118881637

[al45tair]

This is a cherry pick of #8253.

[al45tair]

This isn't needed here any more.