[5.4][Runtime] Fix StableAddressConcurrentReadableHashMap threading crash.

[mikeash]

Cherry-pick #35185 to 5.4.

StableAddressConcurrentReadableHashMap::getOrInsert had a race condition in the first lookup, where the snapshot was destroyed before the pointer was extracted from the returned wrapper. Fix this by creating the snapshot outside the if so that it stays alive.

rdar://problem/71932487

[mikeash]

@swift-ci please test

[mikeash]

@swift-ci please test windows platform

[jckarter]

Sneaky. I wonder if the snapshot's API could all be turned into & methods, so that they're impossible to accidentally use on a temporary snapshot, e.g.:

struct Butt {
  void foo() &;
};

void butt() {
  Butt().foo(); // an error, because the method requires an lvalue but is given a temporary
}