Parse: Don't crash on trailing whitespace before EOF

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.

Differential Revision: http://reviews.llvm.org/D4646