When testing clang that has been compiled with `-DDEFAULT_SYSROOT` set to some path,
some tests would fail. Override sysroot to be empty string for the tests to succeed
when clang is configured with `DEFAULT_SYSROOT`.
Differential Revision: https://reviews.llvm.org/D66834
Patch by Sergej Jaskiewicz <jaskiewiczs@icloud.com>.
llvm-svn: 373147
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