[gn build] (manually) port a7691dee2d (TestAST)

This makes clang/lib/Testing the very first non-test()-target that includes
gtest headers, which means it needs an explicit dep on gtest.

llvm/utils/gn/secondary/clang/lib/Testing/BUILD.gn

@@ -1,6 +1,14 @@
 static_library("Testing") {
   output_name = "clangTesting"
   configs += [ "//llvm/utils/gn/build:clang_code" ]
-  deps = [ "//llvm/lib/Support" ]
-  sources = [ "CommandLineArgs.cpp" ]
+  testonly = true
+  deps = [
+    "//clang/lib/Basic",
+    "//llvm/lib/Support",
+    "//llvm/utils/unittest:gtest",
+  ]
+  sources = [
+    "CommandLineArgs.cpp",
+    "TestAST.cpp",
+  ]
 }

llvm/utils/gn/secondary/clang/unittests/Tooling/BUILD.gn

@@ -11,6 +11,7 @@ unittest("ToolingTests") {
     "//clang/lib/Frontend",
     "//clang/lib/Lex",
     "//clang/lib/Rewrite",
+    "//clang/lib/Testing",
     "//clang/lib/Tooling",
     "//clang/lib/Tooling/Core",
     "//clang/lib/Tooling/DependencyScanning",