Convert the scrutinee of a binding pattern to the right category. (#5662)

When the binding pattern appears within a `var` pattern, convert to a
reference. Otherwise, convert to a value.

This gets the advent of code examples to produce the right answers again
:)

---------

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

toolchain/diagnostics/coverage_test.cpp

@@ -51,6 +51,13 @@ constexpr Kind UntestedKinds[] = {
     // - Require all diagnostics produced by compiling have their first location
     //   be in the file being compiled, never an import.
     Kind::LanguageServerDiagnosticInWrongFile,
+
+    // TODO: This can only fire if we attempt to convert a non-reference
+    // expression to a durable reference binding. At the moment, the only time
+    // we attempt reference binding is within a `var` pattern, where the
+    // conversion cannot fail. This should be covered once we support `ref`
+    // binding syntax.
+    Kind::ConversionFailureNonRefToRef,
 };
 
 // Looks for diagnostic kinds that aren't covered by a file_test.