Compiler option to control SYCL extensions.

Today compiler has only two relevant options to turn on/off SYCL functionality:

| Option | Description |
| -------- | ------------ |
|`-sycl-std=` | SYCL language standard to compile for |
|` -fsycl-unnamed-lambda` | Allow unnamed SYCL lambda kernels |

`-sycl-std` today supports only one value: "1.2.1" (default) and doesn't really help to restrict functionality to SYCL 1.2.1 version.

I think it would be useful if compiler provides more fine grain control over developed SYCL extensions (e.g. unnamed-lambda #387, CTAD extensions #773, USM #256, etc.).

In addition to that, it would be great to align compiler option names with community way.
E.g. should we keep SYCL specific option `-sycl-std` or extend existing `-std` option (e.g. `-std` supports OpenCL standard versions, `hip` and `cuda` values).

## Proposal

1. Extend `-sycl-std=` values with `1.2.1-ext`, which should enable SYCL-1.2.1 functionality with extensions. In addition to that I think this should be a default value similar to how clang enables C++ extensions by default.
1. Add more options controlling extensions. TBD: define which extensions require compiler option. Some extensions do not require compiler support and can be controlled via define (e.g. Intel sub-groups).
1. Align options naming scheme with the clang community to simplify upsteaming.

Thoughts?