[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/Writer/BitcodeWriter.cpp

@@ -630,6 +630,8 @@ static uint64_t getAttrKindEncoding(Attribute::AttrKind Kind) {
     return bitc::ATTR_KIND_COLD;
   case Attribute::Hot:
     return bitc::ATTR_KIND_HOT;
+  case Attribute::ElementType:
+    return bitc::ATTR_KIND_ELEMENTTYPE;
   case Attribute::InaccessibleMemOnly:
     return bitc::ATTR_KIND_INACCESSIBLEMEM_ONLY;
   case Attribute::InaccessibleMemOrArgMemOnly: