[clang] [clang-format] Add DiagHandler for getStyle function (PR #91317)

[Owen Pan via cfe-commits]

================
@@ -1452,6 +1452,35 @@ TEST(ConfigParseTest, GetStyleOfSpecificFile) {
   ASSERT_EQ(*Style, getGoogleStyle());
 }
 
+TEST(ConfigParseTest, GetStyleOutput) {
+    // With output
+    ::testing::internal::CaptureStderr();
+    llvm::vfs::InMemoryFileSystem FS;
+    auto Style = getStyle("{invalid_key=invalid_value}", "a.h", "LLVM", "", &FS, false);
+
+    const std::string output = ::testing::internal::GetCapturedStderr();
+
+    ASSERT_FALSE((bool)Style);
+    ASSERT_FALSE(output.empty());
+    llvm::consumeError(Style.takeError());
----------------
owenca wrote:

```suggestion
  llvm::vfs::InMemoryFileSystem FS;

  // Don't suppress output.
  testing::internal::CaptureStderr();
  auto Style = getStyle("{invalid_key=invalid_value}", "a.h", "LLVM", "", &FS,
                        /*AllowUnknownOptions=*/true);
  const auto output = testing::internal::GetCapturedStderr();
  ASSERT_TRUE((bool)Style);
  ASSERT_FALSE(output.empty());
```

https://github.com/llvm/llvm-project/pull/91317