[libcxx][span] Implement P1976R2

This resolves the NB comment about the construction of a fixed-size span
from a dynamic range.

Differential Revision: https://reviews.llvm.org/D74577

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 6d2599e4f776d0cd88438cb82a00c4fc25cc3f67
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 937683d..77c7c68 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -198,6 +198,8 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_ranges``                              *unimplemented*  
     ------------------------------------------------- -----------------
+    ``__cpp_lib_span``                                ``202002L``      
+    ------------------------------------------------- -----------------
     ``__cpp_lib_three_way_comparison``                *unimplemented*  
     ------------------------------------------------- -----------------
     ``__cpp_lib_to_array``                            ``201907L``