[llvm] TableGen: Generate enum for runtime libcall implementations (PR #144973)

[Sam Clegg via llvm-commits]

sbc100 wrote:

i.e. in the generated `include/llvm/IR/RuntimeLibcalls.inc` file I see:

```
enum Libcall : unsigned short {                                                       
  ACOS_F32 = 0,                                                                       
  ACOS_F64 = 1,                                                                       
  ACOS_F80 = 2,                                                                       
  ACOS_F128 = 3,                                                                      
  ACOS_PPCF128 = 4,   
```

https://github.com/llvm/llvm-project/pull/144973