[Macros] Always parse macro expansions, diagnose later

Always parse macro expansions, regardless of language mode, and
eliminate the fallback path for very, very, very old object literals
like `#Color`. Instead, check for the feature flag for macro
declaration and at macro expansion time, since this is a semantic
restriction.

While here, refactor things so the vast majority of the macro-handling
logic still applies even if the Swift Swift parser is disabled. Only
attempts to expand the macro will fail. This allows us to enable the
macro-diagnostics test everywhere.

test/StringProcessing/Frontend/disable-flag.swift

@@ -9,6 +9,6 @@ _ = /x/
 // expected-error@-2 {{cannot find 'x' in scope}}
 // expected-error@-3 {{'/' is not a postfix unary operator}}
 
-_ = #/x/# // expected-error {{expected expression in assignment}}
+_ = #/x/# // expected-error {{expected a macro identifier}}
 
 func foo(_ x: Regex<Substring>) {} // expected-error {{cannot find type 'Regex' in scope}}