[region-isolation] Add a helper for getting an ApplyIsolationCrossing from an instruction.

I am making this specific API since I am going to make it so that
SILIsolationInfo::get(SILInstruction *) can infer isolation info from self even
from functions that are not apply isolation crossing points. For example, in the
following, we need to understand that test is main actor isolated and we
shouldn't emit an error.

```swift
@mainactor func test(_ x: NonSendable) {}

@OtherActor func doSomething() {
  let x = NonSendable()
  Task.init { @mainactor in print(x) }
  test(x)
}
```

Author: gottesmm

include/swift/SILOptimizer/Analysis/RegionAnalysis.h

@@ -52,6 +52,11 @@ inline bool isNonSendableType(SILType type, SILFunction *fn) {
   return !type.isSendable(fn);
 }
 
+/// Return the ApplyIsolationCrossing for a specific \p inst if it
+/// exists. Returns std::nullopt otherwise.
+std::optional<ApplyIsolationCrossing>
+getApplyIsolationCrossing(SILInstruction *inst);
+
 // This is our PImpl type that we use to hide all of the internal details of
 // the computation.
 class PartitionOpTranslator;

lib/SILOptimizer/Analysis/RegionAnalysis.cpp

@@ -48,6 +48,20 @@ using namespace swift::regionanalysisimpl;
 //                              MARK: Utilities
 //===----------------------------------------------------------------------===//
 
+std::optional<ApplyIsolationCrossing>
+regionanalysisimpl::getApplyIsolationCrossing(SILInstruction *inst) {
+  if (ApplyExpr *apply = inst->getLoc().getAsASTNode<ApplyExpr>())
+    if (auto crossing = apply->getIsolationCrossing())
+      return crossing;
+
+  if (auto fas = FullApplySite::isa(inst)) {
+    if (auto crossing = fas.getIsolationCrossing())
+      return crossing;
+  }
+
+  return {};
+}
+
 namespace {
 
 struct UnderlyingTrackedValueInfo {
@@ -1903,11 +1917,9 @@ class PartitionOpTranslator {
       }
     }
 
-    auto isolationRegionInfo = SILIsolationInfo::get(inst);
-
     // If this apply does not cross isolation domains, it has normal
     // non-transferring multi-assignment semantics
-    if (!bool(isolationRegionInfo))
+    if (!getApplyIsolationCrossing(*fas))
       return translateNonIsolationCrossingSILApply(fas);
 
     if (auto cast = dyn_cast<ApplyInst>(inst))