Use C++14-style return type deduction in LLVM.
Summary:
Simplifies the C++11-style "-> decltype(...)" return-type deduction.
Note that you have to be careful about whether the function return type
is `auto` or `decltype(auto)`. The difference is that bare `auto`
strips const and reference, just like lambda return type deduction. In
some cases that's what we want (or more likely, we know that the return
type is a value type), but whenever we're wrapping a templated function
which might return a reference, we need to be sure that the return type
is decltype(auto).
No functional change.
Subscribers: dexonsmith, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D74383