[CodeCompletion][Parse] Don't drop the contained expression when completing within an InOutExpr

When completing within an InOutExpr like `&foo.<complete>`, we were forming a
CodeCompletionExpr with a base when parsing the content of the InOutExpr and
storing it in CodeCompletionCallbacksImpl::CodeCompleteTokenExpr. When the
result of that parse contained a code completion though, we were dropping the
sub-expression completely and forming an empty code completion result in place
of the inout expression, which resulted in later code inserting a code
completion expression with no base into the AST. The solver based completion
was later asking for the type of the code completion and its base using the
expression stored in CodeCompletionCallbacksImpl::CodeCompleteTokenExpr, but
that never ended up in the AST so we hit an assertion about the expression not
have a type in the formed solutions.

Resolves rdar://75366814

Author: nathawes

lib/Parse/ParseExpr.cpp

@@ -533,7 +533,7 @@ ParserResult<Expr> Parser::parseExprUnary(Diag<> Message, bool isExprBasic) {
 
     ParserResult<Expr> SubExpr = parseExprUnary(Message, isExprBasic);
     if (SubExpr.hasCodeCompletion())
-      return makeParserCodeCompletionResult<Expr>();
+      return makeParserCodeCompletionResult<Expr>(SubExpr.getPtrOrNull());
     if (SubExpr.isNull())
       return nullptr;
     return makeParserResult(

validation-test/IDE/crashers_2_fixed/rdar75366814.swift

@@ -0,0 +1,3 @@
+// RUN: %swift-ide-test -code-completion -code-completion-token=COMPLETE -source-filename %s
+
+if undefined(&self.#^COMPLETE^#