Switch formatv adapters to format_provider (#3434)

The main difference I'm aiming for is that clangd doesn't complain about
the struct being unused, but it does miss the function's use. But really
these are specific to formatv for diagnostics, so this is more clearly
marking such, and probably makes for a better pattern for the future.

toolchain/parse/context.cpp

@@ -23,23 +23,33 @@ enum class RelativeLocation : int8_t {
   Before,
 };
 
+}  // namespace Carbon::Parse
+
+namespace llvm {
+
 // Adapts RelativeLocation for use with formatv.
-// TODO: Investigate for approaches that clangd is happier with.
-static auto operator<<(llvm::raw_ostream& out, RelativeLocation loc)
-    -> llvm::raw_ostream& {
-  switch (loc) {
-    case RelativeLocation::Around:
-      out << "around";
-      break;
-    case RelativeLocation::After:
-      out << "after";
-      break;
-    case RelativeLocation::Before:
-      out << "before";
-      break;
+template <>
+struct format_provider<Carbon::Parse::RelativeLocation> {
+  using RelativeLocation = Carbon::Parse::RelativeLocation;
+  static void format(const RelativeLocation& loc, raw_ostream& out,
+                     StringRef /*style*/) {
+    switch (loc) {
+      case RelativeLocation::Around:
+        out << "around";
+        break;
+      case RelativeLocation::After:
+        out << "after";
+        break;
+      case RelativeLocation::Before:
+        out << "before";
+        break;
+    }
   }
-  return out;
-}
+};
+
+}  // namespace llvm
+
+namespace Carbon::Parse {
 
 Context::Context(Tree& tree, Lex::TokenizedBuffer& tokens,
                  Lex::TokenDiagnosticEmitter& emitter,