[SILVerifier] Add a flag to exit instead of aborting on failure.

This matches what LLVM does, and makes testing/fuzzing easier.
It's also slightly more friendly in case the user violates an
assumption. It can't be turned on by default (yet) because it's
important to crash on verification failures, when running swiftc
in a debugger.

Author: dcci

lib/SIL/SILVerifier.cpp

@@ -50,6 +50,12 @@ static llvm::cl::opt<bool> SkipUnreachableMustBeLastErrors(
                               "verify-skip-unreachable-must-be-last",
                               llvm::cl::init(false));
 
+// This flag controls the default behaviour when hitting a verification
+// failure (abort/exit).
+static llvm::cl::opt<bool> AbortOnFailure(
+                              "verify-abort-on-failure",
+                              llvm::cl::init(true));
+
 // The verifier is basically all assertions, so don't compile it with NDEBUG to
 // prevent release builds from triggering spurious unused variable warnings.
 
@@ -139,7 +145,12 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
       F.print(llvm::dbgs());
     }
 
-    abort();
+    // We abort by default because we want to always crash in
+    // the debugger.
+    if (AbortOnFailure)
+      abort();
+    else
+      exit(1);
   }
 #define require(condition, complaint) \
   _require(bool(condition), complaint ": " #condition)