[code-completion] Ensure consumeDecl() does not alter the state of parser. (#6110)

This also fixes IDE/crashers/029-swift-decl-walk.swift.

Author: nkcsgexi

lib/Parse/ParseDecl.cpp

@@ -1815,6 +1815,7 @@ bool Parser::isStartOfDecl() {
 void Parser::consumeDecl(ParserPosition BeginParserPosition,
                          ParseDeclOptions Flags,
                          bool IsTopLevel) {
+  SourceLoc CurrentLoc = Tok.getLoc();
   backtrackToPosition(BeginParserPosition);
   SourceLoc BeginLoc = Tok.getLoc();
   // Consume tokens up to code completion token.
@@ -1830,6 +1831,9 @@ void Parser::consumeDecl(ParserPosition BeginParserPosition,
                    CurDeclContext, { BeginLoc, EndLoc },
                    BeginParserPosition.PreviousLoc);
 
+  while (SourceMgr.isBeforeInBuffer(Tok.getLoc(), CurrentLoc))
+    consumeToken();
+
   if (IsTopLevel) {
     // Skip the rest of the file to prevent the parser from constructing the
     // AST for it.  Forward references are not allowed at the top level.

validation-test/IDE/crashers/029-swift-decl-walk.swift

@@ -0,0 +1,2 @@
+// RUN: %target-swift-ide-test -code-completion -code-completion-token=A -source-filename=%s
+{var f={{#^A^#}r