[ownership-verifier] Improve error message for owned values leaked due to lack of "lifetime ending uses".

The ownership verifier diagnoses leaks of owned objects in two different
places/ways:

1. If the owned value does not have any "consuming" uses, we bail early without
performing dataflow verification.
2. If the owned value has at least 1 "consuming" uses, we perform dataflow to
determine the blocks where the leaked value escapes without being destroyed.

In the latter case, we give a nice error message saying that a leak occured. In
contrast in the former, we go down a "generic error" path that just says that a
"lifetime ending use" is required. This commit changes that generic error
message for owned parameters to say that the value was leaked, resulting in a
clearer message. The other cases are still left along though.

I also added a test file to specifically test leaks/leak error messages. I added
a few tests to make sure this error message is correct and a few more tests just
for fun.

lib/SIL/SILOwnershipVerifier.cpp

@@ -1921,11 +1921,16 @@ bool SILValueOwnershipChecker::checkValueWithoutLifetimeEndingUses() {
 
   if (!isValueAddressOrTrivial(Value, Mod)) {
     return !handleError([&] {
-      llvm::errs() << "Function: '" << Value->getFunction()->getName() << "'\n"
-                   << "Non trivial values, non address values, and non "
-                      "guaranteed function args must have at least one "
-                      "lifetime ending use?!\n"
-                   << "Value: " << *Value << '\n';
+      llvm::errs() << "Function: '" << Value->getFunction()->getName() << "'\n";
+      if (Value.getOwnershipKind() == ValueOwnershipKind::Owned) {
+        llvm::errs() << "Error! Found a leaked owned value that was never "
+                        "consumed.\n";
+      } else {
+        llvm::errs() << "Non trivial values, non address values, and non "
+                        "guaranteed function args must have at least one "
+                        "lifetime ending use?!\n";
+      }
+      llvm::errs() << "Value: " << *Value << '\n';
     });
   }