Make hasLHS and hasRHS matchers available for ArraySubscriptExpr

Summary:
The hasBase and hasIndex don't tell anything about the position of the
base and the index in the code, so we need hasLHS and hasRHS in some cases.

Reviewers: klimek

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D14212

llvm-svn: 251842