Add _MatchingEngine and _StringProcessing modules.

These modules are part of the experimental declarative string processing feature. If accepted to the Standard Library, _StringProcessing will be available via implicit import just like _Concurrency, though _MatchingEngine will still be hidden as an implementation detail.

`_MatchingEngine` will contain the general-purpose pattern matching engine ISA, bytecode, and executor. `_StringProcessing` will contain regular expression and pattern matching APIs whose implementation depends on the matching engine..

Also consolidates frontend flag `-enable-experimental-regex` as `-enable-experimental-string-processing`.

Resolves rdar://85478647.

lib/Frontend/CompilerInvocation.cpp

@@ -473,8 +473,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   }
 
   // Experimental string processing
-  Opts.EnableExperimentalRegex |=
-      Args.hasArg(OPT_enable_experimental_regex);
+  Opts.EnableExperimentalStringProcessing |=
+      Args.hasArg(OPT_enable_experimental_string_processing);
 
   Opts.DisableAvailabilityChecking |=
       Args.hasArg(OPT_disable_availability_checking);