Implement global option ignoreCache

[hinshun]

• Define option ignoreCache in language/builtin.hlb and run go generate ./gen
• Not sure if global options are supported yet, may need to complete that implementation

Plumbs this LLB option as a global HLB option:

https://github.com/moby/buildkit/blob/21ed8f5c1d078d1e28eac2d443fc791fff60eef9/client/llb/state.go#L412-L414

Usage:

fs foo() {
  image "alpine" with option {
    ignoreCache
  }
  run "echo hi" with option {
    ignoreCache
  }
}