[DiagnoseUnreachable] Ignore dead_end destroys.

Such destroys mark the lifetime end of their operands along their
availability boundary.  They are currently inserted in this test case
by the ClosureLifetimeFixup pass, but in the fullness of time they will
be present for every value which is not explicitly destroyed (that's
what complete OSSA lifetimes is mostly about).

Currently, such destroys are diagnosed by DiagnoseUnreachable.  Fix the
diagnostic pass not to diagnose these valid instructions.

rdar://137960229

Author: nate-chandler

lib/SILOptimizer/Mandatory/DiagnoseUnreachable.cpp

@@ -775,6 +775,14 @@ static bool simplifyBlocksWithCallsToNoReturn(SILBasicBlock &BB,
     if (isa<EndBorrowInst>(currInst))
       return false;
 
+    // destroy_value [dead_end] instructions are inserted at the availability
+    // boundary by lifetime completion.  Such instructions correctly mark the
+    // lifetime boundary of the destroyed value and never arise from dead user
+    // code.
+    auto *dvi = dyn_cast<DestroyValueInst>(currInst);
+    if (dvi && dvi->isDeadEnd())
+      return false;
+
     // If no-return instruction is not something we can point in code or
     // it's an explicit cast, skip it.
     if (!noReturnCall->getLoc().is<RegularLocation>() ||

validation-test/SILOptimizer/rdar137960229.swift

@@ -0,0 +1,9 @@
+// RUN: %target-swift-frontend -c %s -verify
+
+func testFatalError(_ message: @autoclosure () -> String = String()) -> Never {
+  Swift.fatalError()
+}
+
+func test() {
+  testFatalError() // warning: Will never be executed
+}