Improve the diagnostic for #include_next occurring in a file not found
in the include path.

Instead of making the incorrect claim that the included file has an
absolute path, describe the actual pro

Improve the diagnostic for #include_next occurring in a file not found
in the include path.

Instead of making the incorrect claim that the included file has an
absolute path, describe the actual problem: the including file was found
either by absolute path, or relative to such a file, or relative to the
primary source file.

llvm-svn: 356712

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