Parse: Don't crash on trailing whitespace before EOF

[majnemer]

Summary

Parser::ParseDeclarationSpecifiers eagerly updates the source range of
the DeclSpec with the current token position. However, it might not
consume any more tokens.

Fix this by only setting the start of the range, not the end. This way
the SourceRange will be invalid if we don't consume any more tokens.

This fixes PR20413.

[thakis]

Test?

[majnemer]

• Forgot to add the test

[rsmith]

lib/Parse/ParseDecl.cpp

2453

Please add a comment explaining why we only set one of these.

[majnemer]

• Address Richard's review comments.

[majnemer]

Closed by commit rL214018 (authored by @majnemer).

lib/Parse/ParseDecl.cpp

 ///
 void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
                                         const ParsedTemplateInfo &TemplateInfo,
                                         AccessSpecifier AS,
                                         DeclSpecContext DSContext,
                                         LateParsedAttrList *LateAttrs) {
   if (DS.getSourceRange().isInvalid()) {
     DS.SetRangeStart(Tok.getLocation());
-    DS.SetRangeEnd(Tok.getLocation());
+    DS.SetRangeEnd(SourceLocation());
   }
 
   bool EnteringContext = (DSContext == DSC_class || DSContext == DSC_top_level);
   bool AttrsLastTime = false;
   ParsedAttributesWithRange attrs(AttrFactory);
   const PrintingPolicy &Policy = Actions.getASTContext().getPrintingPolicy();
   while (1) {
     bool isInvalid = false;

test/Parser/eof2.cpp

+// RUN: not %clang_cc1 %s -fsyntax-only 2>&1 | FileCheck %s
+
+// CHECK: error: expected expression
+// CHECK: error: expected member name or ';' after declaration specifiers
+// CHECK: error: expected '}'
+// CHECK: note: to match this '{'
+// CHECK: error: expected ';' after class
+// CHECK: 4 errors generated.
+
+// Do not add anything to the end of this file.  This requires the whitespace
+// plus EOF after the '<' token.
+
+template <typename T>
+class a {
+  a<