[IR] Add elementtype attribute

This implements the elementtype attribute specified in D105407. It
just adds the attribute and the specified verifier rules, but
doesn't yet make use of it anywhere.

Differential Revision: https://reviews.llvm.org/D106008

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -1385,6 +1385,8 @@ static Attribute::AttrKind getAttrFromCode(uint64_t Code) {
     return Attribute::Cold;
   case bitc::ATTR_KIND_CONVERGENT:
     return Attribute::Convergent;
+  case bitc::ATTR_KIND_ELEMENTTYPE:
+    return Attribute::ElementType;
   case bitc::ATTR_KIND_INACCESSIBLEMEM_ONLY:
     return Attribute::InaccessibleMemOnly;
   case bitc::ATTR_KIND_INACCESSIBLEMEM_OR_ARGMEMONLY: