[clang][IR] Add support for leaf attribute

This patch adds support for leaf attribute as an optimization hint
in Clang/LLVM.

Differential Revision: https://reviews.llvm.org/D90275

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -646,6 +646,8 @@ static uint64_t getAttrKindEncoding(Attribute::AttrKind Kind) {
     return bitc::ATTR_KIND_NO_ALIAS;
   case Attribute::NoBuiltin:
     return bitc::ATTR_KIND_NO_BUILTIN;
+  case Attribute::NoCallback:
+    return bitc::ATTR_KIND_NO_CALLBACK;
   case Attribute::NoCapture:
     return bitc::ATTR_KIND_NO_CAPTURE;
   case Attribute::NoDuplicate: