This is a `constexpr` alternative to HashString(const char16_t*). We can't make
HashString(const char16_t*) itself `constexpr` because HashUntilZero(const T*)
isn't in a form that older compilers (like GCC 4.9) allow to be made
`constexpr`. (The trick to satisfying those compilers is to use recursion
instead of iteration, to get the function into a single `return` statement.)
This requires making a bunch of other functions `constexpr` as well. It also
requires adding MOZ_{PUSH,POP}_DISABLE_INTEGRAL_CONSTANT_OVERFLOW_WARNING
macros to avoid some MSVC weirdness.
The introduction of RotateLeft5() partly undoes one of the patches from bug
1443342, but that's unavoidable.
This change will help with static allocation of static atoms (bug 1411469).
MozReview-Commit-ID: 7r3PnrQXb29
Nicholas Nethercote
Jeff Walden