[clang][scan-deps] fix new test for readonly work trees

Our setup runs tests with bazel in such a way that the work tree is
readonly, which was causing this test to fail because it couldn't write
the .o file. This fixes that, which was new in 15c3793 when this
test was introduced.

Author: durin42

clang/test/ClangScanDeps/visible-modules.c

@@ -30,7 +30,7 @@
 // RUN: %clang @%t/A.rsp
 
 /// Verify compilation & scan agree with each other.
-// RUN: not %clang @%t/tu.rsp 2>&1 | FileCheck %s --check-prefix=COMPILE
+// RUN: not %clang @%t/tu.rsp -o %t/blah.o 2>&1 | FileCheck %s --check-prefix=COMPILE
 
 // SINGLE:        "visible-clang-modules": [
 // SINGLE-NEXT:     "A"