Attributes: add a new allocalign attribute

This will let us start moving away from hard-coded attributes in
MemoryBuiltins.cpp and put the knowledge about various attribute
functions in the compilers that emit those calls where it probably
belongs.

Differential Revision: https://reviews.llvm.org/D117921

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -610,6 +610,8 @@ static uint64_t getAttrKindEncoding(Attribute::AttrKind Kind) {
   switch (Kind) {
   case Attribute::Alignment:
     return bitc::ATTR_KIND_ALIGNMENT;
+  case Attribute::AllocAlign:
+    return bitc::ATTR_KIND_ALLOC_ALIGN;
   case Attribute::AllocSize:
     return bitc::ATTR_KIND_ALLOC_SIZE;
   case Attribute::AlwaysInline: