[AIX] For XL, pick GCC-compatible std & default warning options

Summary:
LLVM now requires C++14. For IBM XL compilers with C++14 support, this
can be done with the GCC-style options. The relevant block in the CMake
file is split up into smaller parts as part of this patch to allow the
common cases to be shared.

Reviewers: jfb, jasonliu, daltenty, xingxue

Reviewed By: jfb, xingxue

Subscribers: mstorsjo, mgorny, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66256

llvm-svn: 369058