WL#12424: Enable use of C++14 in MySQL server

The aim of this worklog is to enable the use of C++14 in the 8.0 version of
MySQL server.

C++14 is a small extension over C++11 with mostly "bug fixes" and minor 
improvements. Descriptions and code examples of new C++14 features can
be found here:
https://github.com/AnthonyCalandra/modern-cpp-features

With the planned upgrade to Visual Studio 2017, all our current compilers
have proper support for C++14. This means the following minimum version
requirements:
- GCC 5.3 (Linux)
- Clang 4.0 (FreeBSD)
- XCode 9 (MacOS)
- Developer Studio 12.6 (Solaris)
- Visual Studio 2017 (Windows)

NF-1: No changes in behavior or performance regressions should be introduced by
this WL.