[IR] Add support for memory attribute

This implements IR and bitcode support for the memory attribute,
as specified in https://reviews.llvm.org/D135597.

The new attribute is not used for anything yet (and as such, the
old memory attributes are unaffected).

Differential Revision: https://reviews.llvm.org/D135592

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -656,6 +656,8 @@ static uint64_t getAttrKindEncoding(Attribute::AttrKind Kind) {
     return bitc::ATTR_KIND_ALLOCATED_POINTER;
   case Attribute::AllocKind:
     return bitc::ATTR_KIND_ALLOC_KIND;
+  case Attribute::Memory:
+    return bitc::ATTR_KIND_MEMORY;
   case Attribute::Naked:
     return bitc::ATTR_KIND_NAKED;
   case Attribute::Nest: