[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.

lib/AST/Module.cpp

@@ -1407,6 +1407,12 @@ bool ModuleDecl::isStdlibModule() const {
   return !getParent() && getName() == getASTContext().StdlibModuleName;
 }
 
+bool ModuleDecl::hasStandardSubstitutions() const {
+  return !getParent() &&
+      (getName() == getASTContext().StdlibModuleName ||
+       getName() == getASTContext().Id_Concurrency);
+}
+
 bool ModuleDecl::isSwiftShimsModule() const {
   return !getParent() && getName() == getASTContext().SwiftShimsModuleName;
 }