Simplify some implementations of get*Decl.

* NamedDecl and CXXMethodDecl were missing getMostRecentDecl.
* The const version can just forward to the non const.
* getMostRecentDecl can use cast instead of cast_or_null.

This then removes some casts from the callers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193039 91177308-0d34-0410-b5e6-96231b3b80d8