verify-di: Implement DebugInfoVerifier

Implement DebugInfoVerifier, which steals verification relying on
DebugInfoFinder from Verifier.

  - Adds LegacyDebugInfoVerifierPassPass, a ModulePass which wraps
    DebugInfoVerifier.  Uses -verify-di command-line flag.

  - Change verifyModule() to invoke DebugInfoVerifier as well as
    Verifier.

  - Add a call to createDebugInfoVerifierPass() wherever there was a
    call to createVerifierPass().

This implementation as a module pass should sidestep efficiency issues,
allowing us to turn debug info verification back on.

<rdar://problem/15500563>

llvm-svn: 206300

llvm/unittests/Bitcode/BitReaderTest.cpp

@@ -59,6 +59,7 @@ TEST(BitReaderTest, MaterializeFunctionsForBlockAddr) { // PR11677
   std::unique_ptr<Module> m(ModuleOrErr.get());
   PassManager passes;
   passes.add(createVerifierPass());
+  passes.add(createDebugInfoVerifierPass());
   passes.run(*m);
 }