[gn build] add build rule for llvm-locstats

Needed by tests after 30a3652b6a.

llvm/utils/gn/secondary/llvm/test/BUILD.gn

@@ -301,6 +301,7 @@ group("test") {
     "//llvm/utils/TableGen:llvm-tblgen",
     "//llvm/utils/count",
     "//llvm/utils/llvm-lit",
+    "//llvm/utils/llvm-locstats",
     "//llvm/utils/not",
 
     # llvm-config wants libgtest_main.a to exist at runtime when run as in

llvm/utils/gn/secondary/llvm/utils/llvm-locstats/BUILD.gn

@@ -0,0 +1,4 @@
+copy("llvm-locstats") {
+  sources = [ "llvm-locstats.py" ]
+  outputs = [ "$root_out_dir/bin/{{source_name_part}}" ]
+}