[Diagnostics] When checking AssignExpr properly diagnose destination

Currently if destination is unresolved instead of trying to re-typecheck
it again and diagnose structural problems which led to such outcome, it
gets completely ignored in favor of trying to type-check source without
contextual type. That leads to missed diagnostic opportunities, which
results in problems on AST verification and SIL generation stages, and
generally missleading errors e.g. `.x = 0`.

Resolves: SR-3506.

Author: xedin

lib/Sema/CSDiag.cpp

@@ -1990,8 +1990,10 @@ class FailureDiagnosis :public ASTVisitor<FailureDiagnosis, /*exprresult*/bool>{
       TCCOptions options = TCCOptions(),
       ExprTypeCheckListener *listener = nullptr,
       bool allowFreeTypeVariables = true);
-  Expr *typeCheckChildIndependently(Expr *subExpr, TCCOptions options) {
-    return typeCheckChildIndependently(subExpr, Type(), CTP_Unused, options);
+  Expr *typeCheckChildIndependently(Expr *subExpr, TCCOptions options,
+                                    bool allowFreeTypeVariables = true) {
+    return typeCheckChildIndependently(subExpr, Type(), CTP_Unused, options,
+                                       nullptr, allowFreeTypeVariables);
   }
 
   Type getTypeOfTypeCheckedChildIndependently(Expr *subExpr,
@@ -6050,13 +6052,21 @@ bool FailureDiagnosis::visitAssignExpr(AssignExpr *assignExpr) {
 
   auto destType = destExpr->getType();
   if (destType->is<UnresolvedType>() || destType->hasTypeVariable()) {
-    // If we have no useful type information from the destination, just type
+    // Look closer into why destination has unresolved types since such
+    // means that destination has diagnosible structural problems, and it's
+    // better to diagnose destination (if possible) before moving on to
+    // the source of the assignment.
+    destExpr = typeCheckChildIndependently(
+        destExpr, TCC_AllowLValue | TCC_ForceRecheck, false);
+    if (!destExpr)
+      return true;
+
+    // If re-checking destination didn't produce diagnostic, let's just type
     // check the source without contextual information.  If it succeeds, then we
     // win, but if it fails, we'll have to diagnose this another way.
     return !typeCheckChildIndependently(assignExpr->getSrc());
   }
-  
-  
+
   // If the result type is a non-lvalue, then we are failing because it is
   // immutable and that's not a great thing to assign to.
   if (!destType->isLValueType()) {

test/Constraints/assignment.swift

@@ -57,3 +57,4 @@ func f23798944() {
   }
 }
 
+.sr_3506 = 0 // expected-error {{reference to member 'sr_3506' cannot be resolved without a contextual type}}

validation-test/SIL/crashers/034-swift-expr-propagatelvalueaccesskind.sil renamed to validation-test/SIL/crashers_fixed/034-swift-expr-propagatelvalueaccesskind.sil

@@ -1,4 +1,4 @@
-// RUN: not --crash %target-sil-opt %s
+// RUN: not %target-sil-opt %s
 // REQUIRES: asserts
 .x.a=Int
 import Swift

validation-test/compiler_crashers/28368-swift-expr-propagatelvalueaccesskind.swift renamed to validation-test/compiler_crashers_fixed/28368-swift-expr-propagatelvalueaccesskind.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 -typecheck
+// RUN: not %target-swift-frontend %s -typecheck
 guard let f=.h.a=[]{

validation-test/compiler_crashers/28395-swift-expr-propagatelvalueaccesskind.swift renamed to validation-test/compiler_crashers_fixed/28395-swift-expr-propagatelvalueaccesskind.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 -typecheck
+// RUN: not %target-swift-frontend %s -typecheck
 {guard let b=.h.h=.n?(){

validation-test/compiler_crashers/28449-impl-getgraphindex-typevariables-size-out-of-bounds-index-failed.swift renamed to validation-test/compiler_crashers_fixed/28449-impl-getgraphindex-typevariables-size-out-of-bounds-index-failed.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
 for in.E={return $0 c