GB147 15.10 Add a feature-test macro for "consteval" P1902

[wg21bot]

Add a feature-test macro for consteval

Proposed change:
Add __cpp_consteval to Table 17 [tab:cpp.predefined.ft].

[brevzin]

SG10 consensus on the the new macro in D1902R1.

[jensmaurer]

Assigning to EWG for a nod.

[jfbastien]

EWG on Thursday: Forward D1902 to CWG as a resolution to US150, GB147 and GB146, and let SG10 decide names for the macros

SF	F	N	A	SA
16	7	1	0	0

[jensmaurer]

CWG in Belfast: Accepted.