[test] Driver/ftime-trace.cpp: work around -Wmsvc-not-found

There may be one extra warning. Just make the check less strict.
```
clang: warning: unable to find a Visual Studio installation; try running Clang from a developer command prompt [-Wmsvc-not-found]
clang: warning: argument unused during compilation: '-ftime-trace' [-Wunused-command-line-argument]
clang: warning: argument unused during compilation: '-ftime-trace=e' [-Wunused-command-line-argument]
clang: warning: argument unused during compilation: '-ftime-trace-granularity=1' [-Wunused-command-line-argument]
```

clang/test/Driver/ftime-trace.cpp

@@ -55,10 +55,11 @@
 // LINK3: -cc1{{.*}} "-ftime-trace=e{{/|\\\\}}b-{{[^.]*}}.json" "-ftime-trace-granularity=0"
 
 // RUN: %clang -### -ftime-trace -ftime-trace=e -ftime-trace-granularity=1 -xassembler d/a.cpp 2>&1 | \
-// RUN:   FileCheck %s --check-prefix=UNUSED --implicit-check-not=warning:
-// UNUSED: warning: argument unused during compilation: '-ftime-trace'
-// UNUSED: warning: argument unused during compilation: '-ftime-trace=e'
-// UNUSED: warning: argument unused during compilation: '-ftime-trace-granularity=1'
+// RUN:   FileCheck %s --check-prefix=UNUSED
+// UNUSED:      warning: argument unused during compilation: '-ftime-trace'
+// UNUSED-NEXT: warning: argument unused during compilation: '-ftime-trace=e'
+// UNUSED-NEXT: warning: argument unused during compilation: '-ftime-trace-granularity=1'
+// UNUSED-NOT:  warning:
 
 template <typename T>
 struct Struct {