Squelch some warnings.

Fix up some dead code warnings as well as a warning for testing
whether an unsigned number is non-negative (which it always is).

Author: rudkx

include/swift/Basic/OwnedString.h

@@ -52,7 +52,6 @@ class OwnedString {
   void initialize(const char* Data, size_t Length, StringOwnership Ownership) {
     this->Length = Length;
     this->Ownership = Ownership;
-    assert(Length >= 0 && "expected length to be non-negative");
     if (Ownership == StringOwnership::Copied && Data) {
       char *substring = static_cast<char *>(malloc(Length + 1));
       assert(substring && "expected successful malloc of copy");

lib/FrontendTool/FrontendTool.cpp

@@ -391,9 +391,13 @@ class JSONFixitWriter
 // This is a separate function so that it shows up in stack traces.
 LLVM_ATTRIBUTE_NOINLINE
 static void debugFailWithAssertion() {
-  // This assertion should always fail, per the user's request, and should
-  // not be converted to llvm_unreachable.
-  assert(0 && "This is an assertion!");
+  // Per the user's request, this assertion should always fail in
+  // builds with assertions enabled.
+
+  // This should not be converted to llvm_unreachable, as those are
+  // treated as optimization hints in builds where they turn into
+  // __builtin_unreachable().
+  assert((0) && "This is an assertion!");
 }
 
 // This is a separate function so that it shows up in stack traces.

lib/Sema/CSDiag.cpp

@@ -7276,7 +7276,7 @@ bool FailureDiagnosis::visitUnresolvedMemberExpr(UnresolvedMemberExpr *E) {
     case CC_SelfMismatch:         // Self argument mismatches.
     case CC_ArgumentNearMismatch: // Argument list mismatch.
     case CC_ArgumentMismatch:     // Argument list mismatch.
-      assert(0 && "These aren't produced by filterContextualMemberList");
+      llvm_unreachable("These aren't produced by filterContextualMemberList");
       return false;
 
     case CC_ExactMatch: { // This is a perfect match for the arguments.

lib/Sema/TypeCheckSwitchStmt.cpp

@@ -750,7 +750,7 @@ namespace {
           break;
         case SpaceKind::Disjunct: {
           if (forDisplay) {
-            assert(false && "Attempted to display disjunct to user!");
+            llvm_unreachable("Attempted to display disjunct to user!");
           } else {
             buffer << "DISJOIN(";
             for (auto &sp : Spaces) {