[clang] [Sema] Add code completion for if constexpr and consteval (PR #124315)

[Letu Ren via cfe-commits]

FantasqueX wrote:

Hi @HighCommander4 thanks for your suggestions. I'm happy to add snippets support for `if constexpr` and `if consteval`. Since `if 
      <any number white space>  constexp` and `if !consteval` is valid, I decided to add code snippet completion in the specific code completion function instead of in statement completion. Do you think it is OK?

https://github.com/llvm/llvm-project/pull/124315