[CSDiagnostics] Don't crash if we don't have access to the generic param's decl

(cherry picked from commit e50050b)

Author: theblixguy

lib/Sema/CSDiagnostics.cpp

@@ -2958,6 +2958,9 @@ bool MissingGenericArgumentsFailure::diagnoseForAnchor(
   if (!diagnosed)
     return false;
 
+  if (!hasDecl())
+    return true;
+
   auto *DC = getDeclContext();
   if (auto *SD = dyn_cast<SubscriptDecl>(DC)) {
     emitDiagnostic(SD, diag::note_call_to_subscript, SD->getFullName());
@@ -3008,6 +3011,9 @@ bool MissingGenericArgumentsFailure::diagnoseParameter(
     emitDiagnostic(loc, diag::unbound_generic_parameter, GP);
   }
 
+  if (!hasDecl())
+    return true;
+
   if (!hasLoc(GP))
     return true;
 

lib/Sema/CSDiagnostics.h

@@ -1337,6 +1337,8 @@ class MissingGenericArgumentsFailure final : public FailureDiagnostic {
     return GP->getDecl()->getDeclContext();
   }
 
+  bool hasDecl() const { return Parameters.front()->getDecl() != nullptr; }
+
   bool diagnoseAsError() override;
 
   bool diagnoseForAnchor(Anchor anchor,

validation-test/compiler_crashers_2_fixed/sr11236.swift

@@ -0,0 +1,13 @@
+// RUN: not %target-swift-frontend -typecheck %s
+
+public func trichotomy<T : Comparable>(x: T, y: T) -> some Comparable {
+  if x < y { return -1 } 
+  else if x == y { return 0 }
+  return 1
+}
+
+public func myTri<T: Comparable, U: Comparable> (retval: UnsafeMutablePointer<U>, x: UnsafeMutablePointer<T>, y: UnsafeMutablePointer<T>) {
+  retval.initialize(to: trichotomy(x: x, y: y))
+}
+
+print("Hello, World!")