[move-only] Rename mark_must_check -> mark_unresolved_non_copyable_value.

I was originally hoping to reuse mark_must_check for multiple types of checkers.
In practice, this is not what happened... so giving it a name specifically to do
with non copyable types makes more sense and makes the code clearer.

Just a pure rename.

lib/SILOptimizer/Mandatory/MoveOnlyObjectCheckerTester.cpp

@@ -97,9 +97,11 @@ class MoveOnlyObjectCheckerTesterPass : public SILFunctionTransform {
     borrowtodestructure::IntervalMapAllocator allocator;
 
     unsigned diagCount = diagnosticEmitter.getDiagnosticCount();
-    SmallSetVector<MarkMustCheckInst *, 32> moveIntroducersToProcess;
-    bool madeChange = searchForCandidateObjectMarkMustChecks(
-        fn, moveIntroducersToProcess, diagnosticEmitter);
+    SmallSetVector<MarkUnresolvedNonCopyableValueInst *, 32>
+        moveIntroducersToProcess;
+    bool madeChange =
+        searchForCandidateObjectMarkUnresolvedNonCopyableValueInsts(
+            fn, moveIntroducersToProcess, diagnosticEmitter);
 
     LLVM_DEBUG(llvm::dbgs()
                << "Emitting diagnostic when checking for mark must check inst: "