Implement midpoint for floating point types. Reviewed as https://reviews.llvm.org/D61014.

llvm-svn: 359184
diff --git a/libcxx/docs/FeatureTestMacroTable.rst b/libcxx/docs/FeatureTestMacroTable.rst
index 4d3b079..0748c4c 100644
--- a/libcxx/docs/FeatureTestMacroTable.rst
+++ b/libcxx/docs/FeatureTestMacroTable.rst
@@ -188,6 +188,8 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_generic_unordered_lookup``            *unimplemented*  
     ------------------------------------------------- -----------------
+    ``__cpp_lib_interpolate``                         ``201902L``      
+    ------------------------------------------------- -----------------
     ``__cpp_lib_is_constant_evaluated``               ``201811L``      
     ------------------------------------------------- -----------------
     ``__cpp_lib_list_remove_return_type``             *unimplemented*