swift234

Add a method to _NSContiguousString to facilitate stack promotion

Use it for hashing and comparison.

During String's hashValue and comparison function we create a
_NSContiguousString instance to call Foundation's hash/compare function. This is
expensive because we have allocate and deallocate a short lived object on the
heap (and deallocation for Swift objects is expensive).  Instead help the
optimizer to allocate this object on the stack.

Introduces two functions on the internal _NSContiguousString:
_unsafeWithNotEscapedSelfPointer and _unsafeWithNotEscapedSelfPointerPair that
pass the _NSContiguousString instance as an opaque pointer to their closure
argument. Usage of these functions asserts that the closure will not escape
objects transitively reachable from the opaque pointer.

We then use those functions to call into the runtime to call foundation
functions on the passed strings. The optimizer can promote the strings to the
stack because of the assertion this API makes.

  let lhsStr = _NSContiguousString(self._core) // will be promoted to the stack.
  let rhsStr = _NSContiguousString(rhs._core) // will be promoted to the stack.
  let res = lhsStr._unsafeWithNotEscapedSelfPointerPair(rhsStr) {
    return _stdlib_compareNSStringDeterministicUnicodeCollationPointer($0, $1)
  }

Tested by existing String tests.

We should see some nice performance improvements for string comparison and
dictionary benchmarks.

Here is what I measured at -O on my machine

Name                          Speedup
Dictionary                      2.00x
Dictionary2                     1.45x
Dictionary2OfObjects            1.20x
Dictionary3                     1.50x
Dictionary3OfObjects            1.45x
DictionaryOfObjects             1.40x
SuperChars                      1.60x

rdar://22173647