in-progress implementation of the Concepts TS. The recommended feature
test macro __cpp_experimental_concepts is set to 1 (as opposed to
201501) to indicate that the feature is enabled, but the
implementation is incomplete.

The link to the Concepts TS in cxx_status is updated to refer to the
PDTS (N4377). Additional changes related to __has_feature and
__has_extension are to follow in a later change.

Relevant tests include:

test/Lexer/cxx-features.cpp

The test file is updated with testing of the C++14 + Concepts TS mode.
The expected behaviour is the same as that of the C++14 modes except
for the case of __cpp_experimental_concepts."

- Hubert Tong.

Being committed for Hubert (as per his understanding with Richard Smith) as we start work on the concepts-ts following our preliminary strategy session earlier today. 

The patch is tiny and seems quite standard.

Thanks Hubert!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@237982 91177308-0d34-0410-b5e6-96231b3b80d8