P1902 Missing feature-test macros 2017-2019

[wg21bot]

P1902R0 Missing feature-test macros 2018-2019 (Barry Revzin)

[brevzin]

This addresses the following NB comments:

• FI015 FI015 [support.limits.general] Missing feature-testing macros P1902 nbballot#15
• GB146 GB146 15.10 Add a feature-test macro for concepts P1902 nbballot#145
• GB147 GB147 15.10 Add a feature-test macro for "consteval" P1902 nbballot#146
• US150 US150 15.10 [cpp.predefined] Add feature-test macro for "familiar template syntax for generic lambdas" P1902 nbballot#149
• US167 US167 17.03.1 Feature-test macro for non-member ssize() P1902 nbballot#165
• DE168 DE168 17.03.1 Feature-test macros for "constexpr" P1902 nbballot#166

[jensmaurer]

EWG and LEWG are fine with this; assigning to CWG and EWG.

[jensmaurer]

CWG in Belfast: Approved.