[Parse] Fix code completion crash when avoiding skipping

Fix a crash that could occur when performing
completion at the start of an accessor body.
Previously we assumed `CodeCompletion` would never
be null due to function body skipping in the first
pass of code completion. However with the
introduction of the ability to avoid skipping in
certain cases, it might be now be null if we need
to avoid skipping. Found by the stress tester.

rdar://95772803

Author: hamishknight

lib/Parse/ParseDecl.cpp

@@ -7374,7 +7374,8 @@ Parser::parseAbstractFunctionBodyImpl(AbstractFunctionDecl *AFD) {
   // In implicit getter, if a CC token is the first token after '{', it might
   // be a start of an accessor block. Perform special completion for that.
   if (auto accessor = dyn_cast<AccessorDecl>(AFD)) {
-    if (peekToken().is(tok::code_complete) && accessor->isImplicitGetter()) {
+    if (CodeCompletion && peekToken().is(tok::code_complete) &&
+        accessor->isImplicitGetter()) {
       SourceLoc LBraceLoc, RBraceLoc;
       LBraceLoc = consumeToken(tok::l_brace);
       auto *CCE = new (Context) CodeCompletionExpr(Tok.getLoc());

test/SourceKit/CodeComplete/rdar95772803.swift

@@ -0,0 +1,14 @@
+
+var foo: Double { 1 / 2 }
+var bar: Regex<Substring> { /x/ }
+var baz: Regex<Substring> { / x/ }
+var qux: Regex<Substring> { / x}/ }
+
+// Check that we are not crashing
+// RUN: %sourcekitd-test \
+// RUN: -req=complete -pos=2:18 %s -- -enable-bare-slash-regex %s == \
+// RUN: -req=complete -pos=3:28 %s -- -enable-bare-slash-regex %s == \
+// RUN: -req=complete -pos=4:28 %s -- -enable-bare-slash-regex %s == \
+// RUN: -req=complete -pos=5:28 %s -- -enable-bare-slash-regex %s
+
+// REQUIRES: swift_in_compiler