Verifier: Move over DISubprogram::Verify()

Move over the remaining (somewhat complicated) check from
`DISubprogram::Verify()`.  I suspect this check could be optimized --
e.g., it would be nice not to do another full traversal here -- but it's
not exactly obvious how.  For now, just bring it over as is.

Once we have a better model for the "canonical" subprogram of a
`Function`, we should enforce that all `!dbg` attachments lead to the
canonical one.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233663 91177308-0d34-0410-b5e6-96231b3b80d8

unittests/IR/IRBuilderTest.cpp

@@ -17,6 +17,7 @@
 #include "llvm/IR/MDBuilder.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/NoFolder.h"
+#include "llvm/IR/Verifier.h"
 #include "gtest/gtest.h"
 
 using namespace llvm;
@@ -303,8 +304,8 @@ TEST_F(IRBuilderTest, DIBuilder) {
                                   0, true, nullptr);
   auto BadScope = DIB.createLexicalBlockFile(BarSP, File, 0);
   I->setDebugLoc(DebugLoc::get(2, 0, BadScope));
-  EXPECT_FALSE(SP.Verify());
   DIB.finalize();
+  EXPECT_TRUE(verifyModule(*M));
 }