Adding PoisonValue for representing poison value explicitly in IR

Define ConstantData::PoisonValue.
Add support for poison value to LLLexer/LLParser/BitcodeReader/BitcodeWriter.
Add support for poison value to llvm-c interface.
Add support for poison value to OCaml binding.
Add m_Poison in PatternMatch.

Differential Revision: https://reviews.llvm.org/D71126

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -2410,6 +2410,9 @@ Error BitcodeReader::parseConstants() {
     case bitc::CST_CODE_UNDEF:     // UNDEF
       V = UndefValue::get(CurTy);
       break;
+    case bitc::CST_CODE_POISON:    // POISON
+      V = PoisonValue::get(CurTy);
+      break;
     case bitc::CST_CODE_SETTYPE:   // SETTYPE: [typeid]
       if (Record.empty())
         return error("Invalid record");