Sema: Avoid validateDecl() recursion through the 'unsupported existential' check

When type-checking decls, we would ensure they don't reference
existential types formed from protocols with associated types
or 'Self' requirements. However this check was done in both
'stage 1' and 'stage 2', which meant it would be called
recursively from validateDecl().

Fix this by performing the check only once at the end of
type checking a source file.

Author: slavapestov

lib/Sema/TypeCheckDecl.cpp

@@ -3495,8 +3495,7 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
       }
     }
 
-    if ((IsSecondPass && !IsFirstPass) ||
-        decl->getDeclContext()->getAsProtocolOrProtocolExtensionContext()) {
+    if (IsSecondPass && !IsFirstPass) {
       TC.checkUnsupportedProtocolType(decl);
       if (auto nominal = dyn_cast<NominalTypeDecl>(decl)) {
         TC.checkDeclCircularity(nominal);
@@ -4284,8 +4283,10 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
 
     if (IsSecondPass) {
       checkAccessibility(TC, PD);
-      for (auto member : PD->getMembers())
+      for (auto member : PD->getMembers()) {
+        TC.checkUnsupportedProtocolType(member);
         checkAccessibility(TC, member);
+      }
       TC.checkInheritanceClause(PD);
       return;
     }

validation-test/compiler_crashers/28477-anonymous-namespace-declchecker-visitconstructordecl-swift-constructordecl.swift renamed to validation-test/compiler_crashers_fixed/28477-anonymous-namespace-declchecker-visitconstructordecl-swift-constructordecl.swift

@@ -5,5 +5,5 @@
 // See https://swift.org/LICENSE.txt for license information
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
-// RUN: not --crash %target-swift-frontend %s -emit-ir
+// RUN: not %target-swift-frontend %s -emit-ir
 extension A{init(){}typealias e:A{}}protocol A{init(){}typealias e

validation-test/compiler_crashers/28489-this-genericenv-already-have-generic-context.swift renamed to validation-test/compiler_crashers_fixed/28489-this-genericenv-already-have-generic-context.swift

@@ -5,6 +5,6 @@
 // See https://swift.org/LICENSE.txt for license information
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
-// RUN: not --crash %target-swift-frontend %s -emit-ir
+// RUN: not %target-swift-frontend %s -emit-ir
 // REQUIRES: asserts
 guard{protocol a{init()typealias e=a{}typealias e