[tests][Driver] Pass an empty sysroot for `DEFAULT_SYSROOT` builds

[carlocab]

Summary

The baremetal-sysroot test fails when the toolchain is configured with
DEFAULT_SYSROOT. So, to emulate not having passed one at all, let's
pass an empty sysroot instead.

This simply follows D79694, D66834, and D51972. D51972 in particular was
meant to address a failing test for behaviour when --sysroot is not
passed and DEFAULT_SYSROOT is set.

[phosek]

LGTM

[carlocab]

Thanks Petr. I don’t have commit access, so I’d appreciate it if you could commit this for me.

[carlocab]

Per suggestion on Discord, here’s my git author line:

Carlo Cabrera <[e-mail]>

clang/test/Driver/baremetal-sysroot.cpp

 // REQUIRES: shell
 // UNSUPPORTED: system-windows
 
 // Test that when a --sysroot is not provided, driver picks the default
 // location correctly if available.
 
 // RUN: rm -rf %T/baremetal_default_sysroot
 // RUN: mkdir -p %T/baremetal_default_sysroot/bin
 // RUN: mkdir -p %T/baremetal_default_sysroot/lib/clang-runtimes/armv6m-none-eabi
 // RUN: ln -s %clang %T/baremetal_default_sysroot/bin/clang
 
 // RUN: %T/baremetal_default_sysroot/bin/clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
-// RUN:     -target armv6m-none-eabi \
+// RUN:     -target armv6m-none-eabi --sysroot= \
 // RUN:   | FileCheck --check-prefix=CHECK-V6M-C %s
 // CHECK-V6M-C: "{{.*}}clang{{.*}}" "-cc1" "-triple" "thumbv6m-none-unknown-eabi"
 // CHECK-V6M-C-SAME: "-internal-isystem" "{{.*}}/baremetal_default_sysroot{{[/\\]+}}bin{{[/\\]+}}..{{[/\\]+}}lib{{[/\\]+}}clang-runtimes{{[/\\]+}}armv6m-none-eabi{{[/\\]+}}include{{[/\\]+}}c++{{[/\\]+}}v1"
 // CHECk-V6M-C-SAME: "-internal-isystem" "{{.*}}/baremetal_default_sysroot{{[/\\]+}}bin{{[/\\]+}}..{{[/\\]+}}lib{{[/\\]+}}clang-runtimes{{[/\\]+}}armv6m-none-eabi{{[/\\]+}}include"
 // CHECK-V6M-C-SAME: "-x" "c++" "{{.*}}baremetal-sysroot.cpp"
 // CHECK-V6M-C-NEXT: "{{[^"]*}}ld{{(\.(lld|bfd|gold))?}}{{(\.exe)?}}" "{{.*}}.o" "-Bstatic"
 // CHECK-V6M-C-SAME: "-L{{.*}}/baremetal_default_sysroot{{[/\\]+}}bin{{[/\\]+}}..{{[/\\]+}}lib{{[/\\]+}}clang-runtimes{{[/\\]+}}armv6m-none-eabi{{[/\\]+}}lib"
 // CHECK-V6M-C-SAME: "-lc" "-lm" "-lclang_rt.builtins-armv6m"
 // CHECK-V6M-C-SAME: "-o" "{{.*}}.o"