[CodeCompletion] Handle ExprPattern in context type analysis

This improves type inference for code completion in argument position of
EnumElementPattern.

lib/IDE/CodeCompletion.cpp

@@ -4974,6 +4974,18 @@ namespace  {
         Ancestors.pop_back();
       return true;
     }
+
+    std::pair<bool, Pattern *> walkToPatternPre(Pattern *P) override {
+      if (Predicate(P))
+        Ancestors.push_back(P);
+      return { true, P };
+    }
+
+    Pattern *walkToPatternPost(Pattern *P) override {
+      if (Predicate(P))
+        Ancestors.pop_back();
+      return P;
+    }
   };
 } // end anonymous namespace
 
@@ -5020,6 +5032,13 @@ public:
         default:
           return false;
         }
+      } else if (auto P = Node.getAsPattern()) {
+        switch (P->getKind()) {
+        case PatternKind::Expr:
+          return true;
+        default:
+          return false;
+        }
       } else
         return false;
     }) {}
@@ -5060,7 +5079,7 @@ public:
         break;
       }
       default:
-        llvm_unreachable("Unhandled expression kinds.");
+        llvm_unreachable("Unhandled expression kind.");
     }
   }
 
@@ -5085,7 +5104,7 @@ public:
       }
       break;
     default:
-      llvm_unreachable("Unhandled statement kinds.");
+      llvm_unreachable("Unhandled statement kind.");
     }
   }
 
@@ -5127,7 +5146,22 @@ public:
         break;
       }
       default:
-        llvm_unreachable("Unhandled decl kinds.");
+        llvm_unreachable("Unhandled decl kind.");
+    }
+  }
+
+  void analyzePattern(Pattern *P, llvm::function_ref<void(Type)> Callback) {
+    switch (P->getKind()) {
+    case PatternKind::Expr: {
+      auto ExprPat = cast<ExprPattern>(P);
+      if (auto D = ExprPat->getMatchVar()) {
+        if (D->hasInterfaceType())
+          Callback(D->getInterfaceType());
+      }
+      break;
+    }
+    default:
+      llvm_unreachable("Unhandled pattern kind.");
     }
   }
 
@@ -5155,6 +5189,8 @@ public:
         analyzeStmt(Parent, Callback);
       } else if (auto Parent = It->getAsDecl()) {
         analyzeDecl(Parent, Callback);
+      } else if (auto Parent = It->getAsPattern()) {
+        analyzePattern(Parent, Callback);
       }
       if (!PossibleTypes.empty() || !PossibleNames.empty())
         return true;