AST: Cleanup __uuidof related code

Switch some things to use range-based for loops.
Change CXXUuidofExpr::GetUuidAttrOfType's return type to be const.
No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213009 91177308-0d34-0410-b5e6-96231b3b80d8