Implement basic bool and int formatting for diagnostics (#4411)

Note, this supports plurals, but doesn't apply it anywhere. I'm mainly
doing that to demonstrate the approach regarding syntax. See
format_providers.h for details.

toolchain/parse/handle_period.cpp

@@ -2,6 +2,7 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
+#include "toolchain/diagnostics/format_providers.h"
 #include "toolchain/parse/context.h"
 #include "toolchain/parse/handle.h"
 
@@ -36,10 +37,9 @@ static auto HandlePeriodOrArrow(Context& context, NodeKind node_kind,
     return;
   } else {
     CARBON_DIAGNOSTIC(ExpectedIdentifierAfterPeriodOrArrow, Error,
-                      "expected identifier after `{0}`", llvm::StringLiteral);
-    context.emitter().Emit(
-        *context.position(), ExpectedIdentifierAfterPeriodOrArrow,
-        is_arrow ? llvm::StringLiteral("->") : llvm::StringLiteral("."));
+                      "expected identifier after `{0:->|.}`", BoolAsSelect);
+    context.emitter().Emit(*context.position(),
+                           ExpectedIdentifierAfterPeriodOrArrow, is_arrow);
     // If we see a keyword, assume it was intended to be a name.
     // TODO: Should keywords be valid here?
     if (context.PositionKind().is_keyword()) {