[Clang] Prioritise built-in headers, even on musl. #8862
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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
There was a test for the broken header search order that I hadn't spotted. Remove it; we should be using the same search order everywhere. rdar://118881637
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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_nextin 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