Recommit r362649, which was reverted in r262680 because of bugs in std::to_chars (which have now been fixed). Speedup to_string and to_wstring for integers using stack buffer and SSO

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@363003 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed