Modified ArraySubscriptExpr to have accessors getLHS and getRHS in addition

to getBase and getIdx.  getBase and getIdx now return a "normalized" view
of the expression (e.g., always "A[4]" instead of possibly "4[A]").  getLHS
and getRHS return the expressions with syntactic fidelity to the original
source code.

Also modified client code of ArraySubscriptExpr, including the AST dumper
and pretty printer, the return-stack value checker, and the LLVM code
generator.

llvm-svn: 41180