[Concurrency ABI] Add standard substitutions for _Concurrency types.

Introduce a second level of standard substitutions to the mangling,
all of the form `Sc<character>`, and use it to provide standard
substitutions for most of the _Concurrency types.

This is a precursor to rdar://78269642 and a good mangling-size
optimization in its own right.

include/swift/AST/Module.h

@@ -729,6 +729,9 @@ public:
   /// \returns true if this module is the "swift" standard library module.
   bool isStdlibModule() const;
 
+  /// \returns true if this module has standard substitutions for mangling.
+  bool hasStandardSubstitutions() const;
+
   /// \returns true if this module is the "SwiftShims" module;
   bool isSwiftShimsModule() const;