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

llvm-svn: 359184
diff --git a/libcxx/utils/generate_feature_test_macro_components.py b/libcxx/utils/generate_feature_test_macro_components.py
index ed01587..f5e770d 100755
--- a/libcxx/utils/generate_feature_test_macro_components.py
+++ b/libcxx/utils/generate_feature_test_macro_components.py
@@ -565,6 +565,12 @@
    "depends": "!defined(_LIBCPP_HAS_NO_THREADS)",
    "internal_depends": "!defined(_LIBCPP_HAS_NO_THREADS)",
    },
+  {"name": "__cpp_lib_interpolate",
+   "values": {
+     "c++2a": 201902L,
+   },
+   "headers": ["numeric"],
+   },
 ]], key=lambda tc: tc["name"])
 
 def get_std_dialects():