AST: Don't use standard substitutions when mangling typealiases

Swift.UnicodeScalar is a typealias now, and for the TypeDecoder
to work correctly we need to mangle it as a typealias and not as
a struct.

However, we still want to demangle old names that used the
substitution, so don't remove it, instead just skip checking for
a substitution when mangling a typealias.

This does not change the ABI; typealiases are only mangled as
part of debug info.

Author: slavapestov

lib/AST/ASTMangler.cpp

@@ -2174,11 +2174,13 @@ bool ASTMangler::tryAppendStandardSubstitution(const GenericTypeDecl *decl) {
   if (!isStdlibType(decl))
     return false;
 
-  if (char Subst = getStandardTypeSubst(decl->getName().str())) {
-    if (!SubstMerging.tryMergeSubst(*this, Subst, /*isStandardSubst*/ true)) {
-      appendOperator("S", StringRef(&Subst, 1));
+  if (isa<NominalTypeDecl>(decl)) {
+    if (char Subst = getStandardTypeSubst(decl->getName().str())) {
+      if (!SubstMerging.tryMergeSubst(*this, Subst, /*isStandardSubst*/ true)) {
+        appendOperator("S", StringRef(&Subst, 1));
+      }
+      return true;
     }
-    return true;
   }
   return false;
 }

test/DebugInfo/typealias.swift

@@ -63,6 +63,13 @@ func main () {
   // CHECK: [[GENERIC_TYPE]] = !DIDerivedType(tag: DW_TAG_typedef, name: "$s9typealias11NotSpecificaySiGD"
   let g: NotSpecific<Int> = .value
   markUsed(g)
+
+  // Make sure we're not using the abbreviation for this obsolete type that was replaced with a typealias in Swift 4
+  //
+  // CHECK-DAG: !DILocalVariable(name: "h", {{.*}} type: [[UNICODE_SCALAR_TYPE:![0-9]+]])
+  // CHECK: [[UNICODE_SCALAR_TYPE]] = !DIDerivedType(tag: DW_TAG_typedef, name: "$ss13UnicodeScalaraD"
+  let h: UnicodeScalar = "a"
+  markUsed(h)
 }
 
 main()