[CSDiagnostics] Diagnose a key path literal without components early

Detect that diagnose key path literals that do not have any components.

Author: xedin

lib/Sema/CSGen.cpp

@@ -3866,6 +3866,12 @@ namespace {
         base = optTy;
       }
 
+      // If we have a malformed KeyPathExpr e.g. let _: KeyPath<A, C> = \A
+      // let's record a AllowKeyPathMissingComponent fix.
+      if (E->hasSingleInvalidComponent()) {
+        (void)CS.recordFix(AllowKeyPathWithoutComponents::create(CS, locator));
+      }
+
       auto valueLocator =
           CS.getConstraintLocator(E, ConstraintLocator::KeyPathValue);
       auto *value = CS.createTypeVariable(valueLocator, TVO_CanBindToNoEscape |

lib/Sema/CSSimplify.cpp

@@ -12299,13 +12299,9 @@ ConstraintSystem::simplifyKeyPathConstraint(
     if (keyPathTy->isPlaceholder())
       return SolutionKind::Solved;
 
-    // If we have a malformed KeyPathExpr e.g. let _: KeyPath<A, C> = \A
-    // let's record a AllowKeyPathMissingComponent fix.
-    if (keyPath->hasSingleInvalidComponent()) {
-      auto *fix = AllowKeyPathWithoutComponents::create(
-          *this, getConstraintLocator(locator));
-      return recordFix(fix) ? SolutionKind::Error : SolutionKind::Solved;
-    }
+    if (hasFixFor(getConstraintLocator(keyPath),
+                  FixKind::AllowKeyPathWithoutComponents))
+      return SolutionKind::Solved;
 
     // If the root type has been bound to a hole, we cannot infer it.
     if (getFixedTypeRecursive(rootTy, /*wantRValue*/ true)->isPlaceholder())