IR+AArch64: add a "swiftasync" argument attribute.

This extends any frame record created in the function to include that
parameter, passed in X22.

The new record looks like [X22, FP, LR] in memory, and FP is stored with 0b0001
in bits 63:60 (CodeGen assumes they are 0b0000 in normal operation). The effect
of this is that tools walking the stack should expect to see one of three
values there:

  * 0b0000 => a normal, non-extended record with just [FP, LR]
  * 0b0001 => the extended record [X22, FP, LR]
  * 0b1111 => kernel space, and a non-extended record.

All other values are currently reserved.

If compiling for arm64e this context pointer is address-discriminated with the
discriminator 0xc31a and the DB (process-specific) key.

There is also an "i8** @llvm.swift.async.context.addr()" intrinsic providing
front-ends access to this slot (and forcing its creation initialized to nullptr
if necessary).

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -1524,6 +1524,8 @@ static Attribute::AttrKind getAttrFromCode(uint64_t Code) {
     return Attribute::SwiftError;
   case bitc::ATTR_KIND_SWIFT_SELF:
     return Attribute::SwiftSelf;
+  case bitc::ATTR_KIND_SWIFT_ASYNC:
+    return Attribute::SwiftAsync;
   case bitc::ATTR_KIND_UW_TABLE:
     return Attribute::UWTable;
   case bitc::ATTR_KIND_VSCALE_RANGE: