Don't go through the TypeSourceInfo when getting the SourceRange.

VarDecl provides a method getSourceRange(), which provides a more
robust way of getting the SourceRange since the TypeSourceInfo can
be null in certain cases.

Reviewed by: majnemer

Differential Revision: http://reviews.llvm.org/D4281

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211667 91177308-0d34-0410-b5e6-96231b3b80d8