Fix false positive unsequenced access and modification warning in array subscript expression.

Summary: In the [expr.sub] p1, we can read that for a given E1[E2], E1 is sequenced before E2. 

Patch by Mateusz Janek.

Reviewers: rsmith, Rakete1111

Reviewed By: rsmith, Rakete1111

Subscribers: riccibruno, lebedev.ri, Rakete1111, hiraditya, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D50766

llvm-svn: 350874