[AST] Avoid walking a few more expressions in WalkToVarDecls

Avoid walking TapExprs, SingleValueStmtExprs, and key paths. The latter
is important since they can contain invalid VarDecls that will no
longer be visited by the ASTWalker after key path resolution, so we
don't want to create case body vars for them.

Author: hamishknight

lib/AST/Pattern.cpp

@@ -208,10 +208,14 @@ namespace {
       return Action::Continue(P);
     }
 
-    // Only walk into an expression insofar as it doesn't open a new scope -
-    // that is, don't walk into a closure body.
     PreWalkResult<Expr *> walkToExprPre(Expr *E) override {
-      if (isa<ClosureExpr>(E)) {
+      // Only walk into an expression insofar as it doesn't open a new scope -
+      // that is, don't walk into a closure body, TapExpr, or
+      // SingleValueStmtExpr. Also don't walk into key paths since any nested
+      // VarDecls are invalid there, and after being diagnosed by key path
+      // resolution the ASTWalker won't visit them.
+      if (isa<ClosureExpr>(E) || isa<TapExpr>(E) ||
+          isa<SingleValueStmtExpr>(E) || isa<KeyPathExpr>(E)) {
         return Action::SkipNode(E);
       }
       return Action::Continue(E);

validation-test/IDE/crashers_fixed/2a9258ff5360857f.swift

@@ -0,0 +1,6 @@
+// {"kind":"complete","original":"19b878fc","signature":"swift::NamingPatternRequest::evaluate(swift::Evaluator&, swift::VarDecl*) const","signatureAssert":"Assertion failed: (foundVarDecl && \"VarDecl not declared in its parent?\"), function evaluate"}
+// RUN: %target-swift-ide-test -code-completion -batch-code-completion -skip-filecheck -code-completion-diagnostics -source-filename %s
+{
+  guard let \a = answer
+    a
+    #^^#

validation-test/compiler_crashers_2/22119c306663e633.swift renamed to validation-test/compiler_crashers_2_fixed/22119c306663e633.swift

@@ -1,5 +1,5 @@
 // {"signature":"swift::NamingPatternRequest::evaluate(swift::Evaluator&, swift::VarDecl*) const"}
-// RUN: not --crash %target-swift-frontend -typecheck %s
+// RUN: not %target-swift-frontend -typecheck %s
 {
   let a = $0.c ;
   switch a {

validation-test/compiler_crashers_2/7d3e143ce48c791.swift renamed to validation-test/compiler_crashers_2_fixed/7d3e143ce48c791.swift

@@ -1,5 +1,5 @@
 // {"kind":"typecheck","signature":"swift::NamingPatternRequest::evaluate(swift::Evaluator&, swift::VarDecl*) const","signatureAssert":"Assertion failed: (foundVarDecl && \"VarDecl not declared in its parent?\"), function evaluate"}
-// RUN: not --crash %target-swift-frontend -typecheck %s
+// RUN: not %target-swift-frontend -typecheck %s
 {
   if
   case.(let \ a) { a

validation-test/compiler_crashers_2/9ae5dcaffa1a80.swift renamed to validation-test/compiler_crashers_2_fixed/9ae5dcaffa1a80.swift

@@ -1,5 +1,5 @@
 // {"signature":"void (anonymous namespace)::StmtChecker::checkSiblingCaseStmts<swift::CaseStmt* const*>(swift::CaseStmt* const*, swift::CaseStmt* const*, swift::CaseParentKind, bool&, swift::Type)"}
-// RUN: not --crash %target-swift-frontend -typecheck %s
+// RUN: not %target-swift-frontend -typecheck %s
 enum a func b(c : a) -> Int {
   switch
     c {