Driver tests: set `--sysroot=""` to support clang with `DEFAULT_SYSROOT`

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
diff --git a/clang/unittests/Driver/ToolChainTest.cpp b/clang/unittests/Driver/ToolChainTest.cpp
index 80938c8..f84e508 100644
--- a/clang/unittests/Driver/ToolChainTest.cpp
+++ b/clang/unittests/Driver/ToolChainTest.cpp
@@ -60,7 +60,7 @@
                                 llvm::MemoryBuffer::getMemBuffer("\n"));
 
   std::unique_ptr<Compilation> C(TheDriver.BuildCompilation(
-      {"-fsyntax-only", "--gcc-toolchain=", "foo.cpp"}));
+      {"-fsyntax-only", "--gcc-toolchain=", "--sysroot=", "foo.cpp"}));
   EXPECT_TRUE(C);
 
   std::string S;