[llvm] [libsycl] Add constructors for context (PR #214538)

[Kseniya Tikhomirova via llvm-commits]

KseniyaTikhomirova wrote:

> Ok, I've addressed the reviews, regarding the doxygen documentation, please let me know what you think of it, I've omitted all self explanatory explicit `\param` in favor of `\p` in brief, for propList, asyncHandler, I've omitted them completely as they would have added exact boilerplate documentation that we try to get rid of (I think). I've only applied these changes to lines of code added by the PR, if this is accepted, I will open a follow-up ticket for the rest of the sycl codebase, changes will most likely be fragmented in smaller PRs

last version seems to be ok. let's keep it for now. 
I would like to study doxygen features, LLVM recommendations and best practices more and create a doc for libsycl with guidance how to document the project. So please hold on a little bit with update of other classes. 

https://github.com/llvm/llvm-project/pull/214538