[DiagnosticInfo] Allow std::string_view in DiagnosticBuilder operator<<. (#190374)

After a68ae7b0cc0922b79114aabe8cf1ec8dc68524d7, calling `<<` with a
`std::string_view` gives:

```
clang/include/clang/Basic/Diagnostic.h:1319:8: error: use of overloaded operator '<<' is ambiguous (with operand types 'const StreamingDiagnostic' and 'const std::string_view')
 1319 |     DB << V;
      |     ~~ ^  ~
```

clang/include/clang/Basic/Diagnostic.h

@@ -36,6 +36,7 @@
 #include <memory>
 #include <optional>
 #include <string>
+#include <string_view>
 #include <type_traits>
 #include <utility>
 #include <vector>
@@ -1374,6 +1375,12 @@ inline const StreamingDiagnostic &operator<<(const StreamingDiagnostic &DB,
   return DB;
 }
 
+inline const StreamingDiagnostic &operator<<(const StreamingDiagnostic &DB,
+                                             std::string_view S) {
+  DB.AddString(S);
+  return DB;
+}
+
 inline const StreamingDiagnostic &operator<<(const StreamingDiagnostic &DB,
                                              const std::string &S) {
   DB.AddString(S);

clang/unittests/Basic/DiagnosticTest.cpp

@@ -23,6 +23,7 @@
 #include "gtest/gtest.h"
 #include <memory>
 #include <optional>
+#include <string_view>
 #include <vector>
 
 using namespace llvm;
@@ -42,8 +43,19 @@ void clang::DiagnosticsTestHelper(DiagnosticsEngine &diag) {
 namespace {
 using testing::AllOf;
 using testing::ElementsAre;
+using testing::HasSubstr;
 using testing::IsEmpty;
 
+MATCHER_P(WithMessage, M,
+          "has diagnostic message that " +
+              ::testing::DescribeMatcher<std::string>(M)) {
+  return testing::ExplainMatchResult(M, arg.getMessage().str(),
+                                     result_listener);
+}
+MATCHER(IsError, "has error severity") {
+  return arg.getLevel() == DiagnosticsEngine::Level::Error;
+}
+
 // Check that DiagnosticErrorTrap works with SuppressAllDiagnostics.
 TEST(DiagnosticTest, suppressAndTrap) {
   DiagnosticOptions DiagOpts;
@@ -164,20 +176,20 @@ TEST(DiagnosticTest, diagnosticError) {
   EXPECT_EQ(Value->first, 20);
 }
 
+class CaptureDiagnosticConsumer : public DiagnosticConsumer {
+public:
+  SmallVector<StoredDiagnostic> StoredDiags;
+
+  void HandleDiagnostic(DiagnosticsEngine::Level level,
+                        const Diagnostic &Info) override {
+    StoredDiags.push_back(StoredDiagnostic(level, Info));
+  }
+};
+
 TEST(DiagnosticTest, storedDiagEmptyWarning) {
   DiagnosticOptions DiagOpts;
   DiagnosticsEngine Diags(DiagnosticIDs::create(), DiagOpts);
 
-  class CaptureDiagnosticConsumer : public DiagnosticConsumer {
-  public:
-    SmallVector<StoredDiagnostic> StoredDiags;
-
-    void HandleDiagnostic(DiagnosticsEngine::Level level,
-                          const Diagnostic &Info) override {
-      StoredDiags.push_back(StoredDiagnostic(level, Info));
-    }
-  };
-
   CaptureDiagnosticConsumer CaptureConsumer;
   Diags.setClient(&CaptureConsumer, /*ShouldOwnClient=*/false);
   Diags.Report(diag::pp_hash_warning) << "";
@@ -187,6 +199,21 @@ TEST(DiagnosticTest, storedDiagEmptyWarning) {
   Diags.Report(CaptureConsumer.StoredDiags.front());
 }
 
+// std::string_view is used by downstream consumers.
+TEST(DiagnosticTest, reportAcceptsStringViewMessage) {
+  DiagnosticOptions DiagOpts;
+  DiagnosticsEngine Diags(DiagnosticIDs::create(), DiagOpts);
+
+  CaptureDiagnosticConsumer CaptureConsumer;
+  Diags.setClient(&CaptureConsumer, /*ShouldOwnClient=*/false);
+
+  std::string_view SV = "diagnostic";
+  Diags.Report(diag::err_target_unknown_triple) << SV;
+
+  EXPECT_THAT(CaptureConsumer.StoredDiags,
+              ElementsAre(WithMessage(HasSubstr("diagnostic"))));
+}
+
 class SuppressionMappingTest : public testing::Test {
 public:
   SuppressionMappingTest() {
@@ -226,13 +253,6 @@ private:
   CaptureDiagnosticConsumer CaptureConsumer;
 };
 
-MATCHER_P(WithMessage, Msg, "has diagnostic message") {
-  return arg.getMessage() == Msg;
-}
-MATCHER(IsError, "has error severity") {
-  return arg.getLevel() == DiagnosticsEngine::Level::Error;
-}
-
 TEST_F(SuppressionMappingTest, MissingMappingFile) {
   Diags.getDiagnosticOptions().DiagnosticSuppressionMappingsFile = "foo.txt";
   clang::ProcessWarningOptions(Diags, Diags.getDiagnosticOptions(), *FS);