Features: Split the Clang testable features to a separate file.

These tests can be shared with the AppleClang compile feature tests.
Features: Use the correct dialect flag when recording features.

Avoid using -std=c++1y for compilers which support -std=c++14, for
example.
Merge topic 'compile-features-refactor' into next

462ca60a Features: Use the correct dialect flag when recording features.
e68d5c60 Features: Split the Clang testable features to a separate file.
Features: Run GNU feature tests with std=c++14 when available.
Merge topic 'compile-features-refactor' into next

9d3102ca Features: Run GNU feature tests with std=c++14 when available.