Remove uses of StringLiteral in format strings. (#4416)

Building on #4411, avoid using StringLiteral in format strings. This
includes a diagnostic check to prevent regressions (which is also how I
gathered issues).

Note, I haven't looked at `std::string` uses yet, but we might need
things like that to be able to pass strings in code back to the user.
StringLiteral though means that it's literally written down in the
toolchain, at which point it should probably be written in the format
string instead of separately.

---------

Co-authored-by: Geoff Romer <gromer@google.com>

toolchain/parse/handle_binding_pattern.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"
 
@@ -25,12 +26,13 @@ auto HandleBindingPattern(Context& context) -> void {
   }
 
   // Handle an invalid pattern introducer for parameters and variables.
-  auto on_error = [&](llvm::StringLiteral expected) {
+  auto on_error = [&](bool expected_name) {
     if (!state.has_error) {
       CARBON_DIAGNOSTIC(ExpectedBindingPattern, Error,
-                        "expected {0} in binding pattern", llvm::StringLiteral);
+                        "expected {0:name|`:` or `:!`} in binding pattern",
+                        BoolAsSelect);
       context.emitter().Emit(*context.position(), ExpectedBindingPattern,
-                             expected);
+                             expected_name);
       state.has_error = true;
     }
   };
@@ -51,7 +53,7 @@ auto HandleBindingPattern(Context& context) -> void {
     // Add a placeholder for the name.
     context.AddLeafNode(NodeKind::IdentifierName, *context.position(),
                         /*has_error=*/true);
-    on_error("name");
+    on_error(/*expected_name=*/true);
   }
 
   if (auto kind = context.PositionKind();
@@ -64,7 +66,7 @@ auto HandleBindingPattern(Context& context) -> void {
     context.PushState(state);
     context.PushStateForExpr(PrecedenceGroup::ForType());
   } else {
-    on_error("`:` or `:!`");
+    on_error(/*expected_name=*/false);
     // Add a placeholder for the type.
     context.AddLeafNode(NodeKind::InvalidParse, *context.position(),
                         /*has_error=*/true);