[llvm] [TableGen, CodeGen, CHERI] Add support for the cPTR wildcard value type. (PR #158426)

[Andrew Adams via llvm-commits]

abadams wrote:

Oddly (if git bisect isn't lying to me), this commit makes `llvm::ARMAsmPrinter::emitAttributes` make a 1.5 MB stack allocation and overflow the stack on many platforms, causing widespread issues for Halide. Before this commit there was still a 0.5 MB stack allocation, which seems large, so I think this just tickles an existing problem in a way that makes it worse, rather than being the true cause. Here's the function preamble before this commit (at 0c1087b377b49dafaa7d82942db11c2fd3fd4c11)

```   
   0:	f3 0f 1e fa          	endbr64
   4:	55                   	push   %rbp
   5:	48 89 e5             	mov    %rsp,%rbp
   8:	41 57                	push   %r15
   a:	41 56                	push   %r14
   c:	41 55                	push   %r13
   e:	41 54                	push   %r12
  10:	53                   	push   %rbx
  11:	4c 8d 9c 24 00 a0 f8 	lea    -0x76000(%rsp),%r11
  18:	ff 
  19:	48 81 ec 00 10 00 00 	sub    $0x1000,%rsp
  20:	48 83 0c 24 00       	orq    $0x0,(%rsp)
  25:	4c 39 dc             	cmp    %r11,%rsp
  28:	75 ef                	jne    19 <_ZN4llvm13ARMAsmPrinter14emitAttributesEv+0x19>
```

It's subtracting 0x76000 from the stack pointer in a loop, touching each page once

and here it is after:

```
   0:	f3 0f 1e fa          	endbr64
   4:	55                   	push   %rbp
   5:	48 89 e5             	mov    %rsp,%rbp
   8:	41 57                	push   %r15
   a:	41 56                	push   %r14
   c:	41 55                	push   %r13
   e:	41 54                	push   %r12
  10:	53                   	push   %rbx
  11:	4c 8d 9c 24 00 30 e8 	lea    -0x17d000(%rsp),%r11
  18:	ff 
  19:	48 81 ec 00 10 00 00 	sub    $0x1000,%rsp
  20:	48 83 0c 24 00       	orq    $0x0,(%rsp)
  25:	4c 39 dc             	cmp    %r11,%rsp
  28:	75 ef                	jne    19 <_ZN4llvm13ARMAsmPrinter14emitAttributesEv+0x19>

```

It's the same, but now it subtracts 0x17d000 from the stack pointer

https://github.com/llvm/llvm-project/pull/158426