[clang] -fimplicit-constexpr release notes.

Author: hanickadot

Diff for: clang/docs/ReleaseNotes.rst

@@ -211,6 +211,8 @@ New Compiler Flags
   The feature has `existed <https://clang.llvm.org/docs/SourceBasedCodeCoverage.html#running-the-instrumented-program>`_)
   for a while and this is just a user facing option.
 
+- New option ``-fimplicit-constexpr`` which implicitly makes all inlined functions implicitly ``constexpr``.
+
 Deprecated Compiler Flags
 -------------------------