[Format] Make it easy to add new format::FormatStyle::LanguageStandard. NFCI

Preparatory change for D65043.

We current use `!=LS_Cpp03` to enable language standards 11,14,17, and
2a. `>=LS_Cpp11` is better if we decide to add new LanguageStandard in
the future.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D65183

llvm-svn: 366876