[llvm] [InlineAsm] refactor InlineAsm class NFC (PR #65649)

[Nathaniel McVicar via llvm-commits]

NathanielMcVicar wrote:

This change appears to have broken the MLIR Windows buildbot (and probably some other Windows bots).
https://lab.llvm.org/buildbot/#/builders/13/builds/39966
```
C:\buildbot\mlir-x64-windows-ninja\llvm-project\llvm\include\llvm\IR\InlineAsm.h(347) : error C2220: the following warning is treated as an error
C:\buildbot\mlir-x64-windows-ninja\llvm-project\llvm\include\llvm\IR\InlineAsm.h(347) : warning C4715: 'llvm::InlineAsm::Flag::getKindName': not all control paths return a value
```

https://github.com/llvm/llvm-project/pull/65649