[STLExtras] Make const the * operator for mapped_iterator.

Summary:
The current non-const * operator shadows the const operator in
iterator_adaptor_base.

Reviewers: mehdi_amini, rriddle!, dblaikie, timshen

Subscribers: dexonsmith, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, Joonsoo, grosul1, frgossen, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78073