Improve clarity/consistency of a few UsingDecl methods and related helpers.
No functionality change.

In Sema helper functions:
 * renamed isTypeName as HasTypenameKeyword
In UsingDecl:
 * renamed get/setUsingLocation to get/setUsingLoc
 * renamed is/setTypeName as has/setTypename



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186816 91177308-0d34-0410-b5e6-96231b3b80d8