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.

stdlib/public/StringProcessing/Regex.swift

@@ -0,0 +1,8 @@
+import Swift
+import _MatchingEngine
+
+public struct DummyRegex<Match> {
+  public init() {
+    fatalError("Unimplemented")
+  }
+}