[tests][Driver] Set `--sysroot=""` to allow `DEFAULT_SYSROOT` build

[hubert.reinterpretcast]

Summary

If DEFAULT_SYSROOT is configured to some path, some tests would fail. This patch overrides sysroot to be the empty string in the style of D66834 so that the tests will pass even when the build is configured with a DEFAULT_SYSROOT.

[mstorsjo]

LGTM

clang/test/Driver/darwin-header-search-libcxx.cpp

 
 // Check with only headers alongside the installation (those should be used,
 // but we should still add /usr/include/c++/v1 after to preserve legacy).
 //
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
 // RUN:     -target x86_64-apple-darwin \
 // RUN:     -stdlib=libc++ \
 // RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin \
+// RUN:     --sysroot="" \
 // RUN:   | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain --check-prefix=CHECK-LIBCXX-TOOLCHAIN-1 %s
 // CHECK-LIBCXX-TOOLCHAIN-1: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 // CHECK-LIBCXX-TOOLCHAIN-1: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
 // CHECK-LIBCXX-TOOLCHAIN-1: "-internal-isystem" "/usr/include/c++/v1"
 //
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
 // RUN:     -target x86_64-apple-darwin \
 // RUN:     -stdlib=libc++ \

clang/test/Driver/darwin-header-search-system.cpp

 // CHECK-NOSTDINC-NOT: "-internal-externc-isystem" "[[SYSROOT]]/usr/include"
 
 // Check search paths without -isysroot
 //
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
 // RUN:     -target x86_64-apple-darwin \
 // RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain_no_libcxx/usr/bin \
 // RUN:     -resource-dir=%S/Inputs/resource_dir \
+// RUN:     --sysroot="" \
 // RUN:   | FileCheck -DRESOURCE=%S/Inputs/resource_dir \
 // RUN:               --check-prefix=CHECK-NOSYSROOT %s
 // CHECK-NOSYSROOT: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 // CHECK-NOSYSROOT: "-internal-isystem" "/usr/local/include"
 // CHECK-NOSYSROOT: "-internal-isystem" "[[RESOURCE]]/include"
 // CHECK-NOSYSROOT: "-internal-externc-isystem" "/usr/include"
 
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \

clang/test/Driver/mingw-sysroot.cpp

 // RUN: mkdir -p %T/testroot-clang/bin
 // RUN: ln -s %clang %T/testroot-clang/bin/x86_64-w64-mingw32-clang
 // RUN: ln -s %S/Inputs/mingw_ubuntu_posix_tree/usr/x86_64-w64-mingw32 %T/testroot-clang/x86_64-w64-mingw32
 
 
 // If we find a gcc in the path with the right triplet prefix, pick that as
 // sysroot:
 
-// RUN: env "PATH=%T/testroot-gcc/bin:%PATH%" %clang -target x86_64-w64-mingw32 -rtlib=platform -stdlib=libstdc++ -c -### %s 2>&1 | FileCheck -check-prefix=CHECK_TESTROOT_GCC %s
+// RUN: env "PATH=%T/testroot-gcc/bin:%PATH%" %clang -target x86_64-w64-mingw32 -rtlib=platform -stdlib=libstdc++ --sysroot="" -c -### %s 2>&1 | FileCheck -check-prefix=CHECK_TESTROOT_GCC %s
 // CHECK_TESTROOT_GCC: "{{.*}}/testroot-gcc{{/|\\\\}}lib{{/|\\\\}}gcc{{/|\\\\}}x86_64-w64-mingw32{{/|\\\\}}5.3-posix{{/|\\\\}}include{{/|\\\\}}c++"
 // CHECK_TESTROOT_GCC: "{{.*}}/testroot-gcc{{/|\\\\}}lib{{/|\\\\}}gcc{{/|\\\\}}x86_64-w64-mingw32{{/|\\\\}}5.3-posix{{/|\\\\}}include{{/|\\\\}}c++{{/|\\\\}}x86_64-w64-mingw32"
 // CHECK_TESTROOT_GCC: "{{.*}}/testroot-gcc{{/|\\\\}}lib{{/|\\\\}}gcc{{/|\\\\}}x86_64-w64-mingw32{{/|\\\\}}5.3-posix{{/|\\\\}}include{{/|\\\\}}c++{{/|\\\\}}backward"
 // CHECK_TESTROOT_GCC: "{{.*}}/testroot-gcc{{/|\\\\}}x86_64-w64-mingw32{{/|\\\\}}include"
 
 
 // If there's a matching sysroot next to the clang binary itself, prefer that
 // over a gcc in the path:
 
-// RUN: env "PATH=%T/testroot-gcc/bin:%PATH%" %T/testroot-clang/bin/x86_64-w64-mingw32-clang -target x86_64-w64-mingw32 -rtlib=compiler-rt -stdlib=libstdc++ -c -### %s 2>&1 | FileCheck -check-prefix=CHECK_TESTROOT_CLANG %s
+// RUN: env "PATH=%T/testroot-gcc/bin:%PATH%" %T/testroot-clang/bin/x86_64-w64-mingw32-clang -target x86_64-w64-mingw32 -rtlib=compiler-rt -stdlib=libstdc++ --sysroot="" -c -### %s 2>&1 | FileCheck -check-prefix=CHECK_TESTROOT_CLANG %s
 // CHECK_TESTROOT_CLANG: "{{.*}}/testroot-clang{{/|\\\\}}x86_64-w64-mingw32{{/|\\\\}}include"
 
 
 // If we pick a root based on a sysroot next to the clang binary, which also
 // happens to be in the same directory as gcc, make sure we still can pick up
 // the libgcc directory:
 
-// RUN: env "PATH=%T/testroot-gcc/bin:%PATH%" %T/testroot-gcc/bin/x86_64-w64-mingw32-clang -target x86_64-w64-mingw32 -rtlib=platform -stdlib=libstdc++ -c -### %s 2>&1 | FileCheck -check-prefix=CHECK_TESTROOT_GCC %s
+// RUN: env "PATH=%T/testroot-gcc/bin:%PATH%" %T/testroot-gcc/bin/x86_64-w64-mingw32-clang -target x86_64-w64-mingw32 -rtlib=platform -stdlib=libstdc++ --sysroot="" -c -### %s 2>&1 | FileCheck -check-prefix=CHECK_TESTROOT_GCC %s