[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/Reader/BitcodeReader.cpp

@@ -1433,6 +1433,8 @@ static Attribute::AttrKind getAttrFromCode(uint64_t Code) {
     return Attribute::NoAlias;
   case bitc::ATTR_KIND_NO_BUILTIN:
     return Attribute::NoBuiltin;
+  case bitc::ATTR_KIND_NO_CALLBACK:
+    return Attribute::NoCallback;
   case bitc::ATTR_KIND_NO_CAPTURE:
     return Attribute::NoCapture;
   case bitc::ATTR_KIND_NO_DUPLICATE: