[gn build] Use `=` for of -fdebug-compilation-dir

-f flags usually use the `=` form. -fdebug-compilation-dir= has been
around for a few months now (since 0c2bb6b446, both LLVM 12.0
and 13.0 have it), so using it shouldn't be a big problem -- especially
since use_relative_paths_in_debug_info is opt-in anyways.

llvm/utils/gn/build/BUILD.gn

@@ -262,10 +262,7 @@ config("compiler_defaults") {
       }
     }
     if (use_relative_paths_in_debug_info) {
-      cflags += [
-        "-fdebug-compilation-dir",
-        ".",
-      ]
+      cflags += [ "-fdebug-compilation-dir=." ]
     }
   }
   if (sysroot != "") {