[SILVerifier] Log on bad debug-info.

Previously, logging of the actually problematic instruction was guarded
by LLVM_DEBUG.  Meanwhile the verifier's require method prints an
instruction (usually one different from that at which the non-contiguous
scope was encountered).

Here, instead, the problematic instruction and the instruction which
defined the previous scope are printed to llvm::errs always (i.e.
whenever verification is actually run).

Additionally, debug-info logging is forcibly set on upon failure so that
the logs clearly show both what the previous scope was, what the current
scope is, and what instructions defined them.

Author: nate-chandler

lib/SIL/Verifier/SILVerifier.cpp

@@ -79,6 +79,7 @@ static llvm::cl::opt<bool> SkipConvertEscapeToNoescapeAttributes(
 // Allow unit tests to gradually migrate toward -allow-critical-edges=false.
 static llvm::cl::opt<bool> AllowCriticalEdges("allow-critical-edges",
                                               llvm::cl::init(true));
+extern llvm::cl::opt<bool> SILPrintDebugInfo;
 
 // The verifier is basically all assertions, so don't compile it with NDEBUG to
 // prevent release builds from triggering spurious unused variable warnings.
@@ -5801,6 +5802,7 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
       return;
 
     const SILDebugScope *LastSeenScope = nullptr;
+    SILInstruction *LastSeenScopeInst = nullptr;
     for (SILInstruction &SI : *BB) {
       if (SI.isMetaInstruction())
         continue;
@@ -5814,6 +5816,7 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
       if (!AlreadySeenScopes.count(DS)) {
         AlreadySeenScopes.insert(DS);
         LastSeenScope = DS;
+        LastSeenScopeInst = &SI;
         continue;
       }
 
@@ -5838,13 +5841,24 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
 
       if (isAncestorScope(DS, LastSeenScope)) {
         LastSeenScope = DS;
+        LastSeenScopeInst = &SI;
         continue;
       }
       if (DS != LastSeenScope) {
-        LLVM_DEBUG(llvm::dbgs() << "Broken instruction!\n"; SI.dump());
-        LLVM_DEBUG(llvm::dbgs() << "Please report a bug on bugs.swift.org\n");
-        LLVM_DEBUG(llvm::dbgs() <<
-          "Pass -Xllvm -verify-di-holes=false to disable the verification\n");
+        llvm::errs() << "Broken instruction!\n"; 
+        SI.dump();
+        llvm::errs() << "in scope\n";
+        DS->print(SI.getFunction()->getModule());
+        llvm::errs() << "Previous, non-contiguous scope set by";
+        LastSeenScopeInst->dump();
+        llvm::errs() << "in scope\n";
+        LastSeenScope->print(SI.getFunction()->getModule());
+        llvm::errs() << "Please report a bug on bugs.swift.org\n";
+        llvm::errs() <<
+          "Pass -Xllvm -verify-di-holes=false to disable the verification\n";
+        // Turn on debug info printing so that the log actually shows the bad
+        // scopes.
+        SILPrintDebugInfo.setValue(true);
         require(
             DS == LastSeenScope,
             "Basic block contains a non-contiguous lexical scope at -Onone");