[SYCL] Make invoke_simd convert its arguments to appropriate type.

- Introduce an intermediate lambda in invoke_simd and call SIMD target
from the lambda with given (SIMD) arguments coming from the lambda's
formal paremeters. This way compiler automatically performs necessary
argument type conversion.

- Introduce a new implicit constructor for simd to allow conversion
of simd objects with _VecExt storage kind (used in invoke_simd
extension).

- Fix linkonce_odr linkage of entry points : change it to external
  linkage to avoid removal by the inliner and llvm-linker.

This new scheme complicates the SIMD target function pointer flow, so
LowerInvokeSimd.cpp update is needed to accomodate the new flow.

Signed-off-by: Konstantin S Bobrovsky <[email protected]>

Author: kbobrovs