[libc++] Implement format_error.
This is the first step at implementing <format>. It adds the <format> header
and implements the `format_error`. class.
Implemnts parts of:
-P0645 Text Formatting
Reviewed By: ldionne, #libc, miscco, curdeius
Differential Revision: https://reviews.llvm.org/D92214
GitOrigin-RevId: 081c1db02dd2b48cf736d44862f7d7e8c2869577
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index ed05488..5fe7005 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -234,6 +234,8 @@
------------------------------------------------- -----------------
``__cpp_lib_execution`` *unimplemented*
------------------------------------------------- -----------------
+ ``__cpp_lib_format`` *unimplemented*
+ ------------------------------------------------- -----------------
``__cpp_lib_generic_unordered_lookup`` ``201811L``
------------------------------------------------- -----------------
``__cpp_lib_int_pow2`` ``202002L``