The `sycl_kernel_entry_point` attribute is used to declare a function that
defines a pattern for an offload kernel to be emitted. The attribute requires
a single type argument that specifies the type used as a SYCL kernel name as
described in section 5.2, "Naming of kernels", of the SYCL 2020 specification.

Properties of the offload kernel are collected when a function declared with
the `sycl_kernel_entry_point` attribute is parsed or instantiated. These
properties, such as the kernel name type, are stored in the AST context where
they are (or will be) used for diagnostic purposes and to facilitate reflection
to a SYCL run-time library. These properties are not serialized with the AST
but are recreated upon deserialization.

The `sycl_kernel_entry_point` attribute is intended to replace the existing
`sycl_kernel` attribute which is intended to be deprecated in a future change
and removed following an appropriate deprecation period. The new attribute
differs in that it is enabled for both SYCL host and device compilation, may
be used with non-template functions, explicitly indicates the type used as
the kernel name type, and will impact AST generation.

This change adds the basic infrastructure for the new attribute. Future
changes will add diagnostics and new AST support that will be used to drive
generation of the corresponding offload kernel.

Changes include:
- Removal of an unneeded inclusion of <string>.
- An update to the `sycl_kernel_entry_point` attribute documentation.
- Updates to the AST test to validate next line proximity where applicable.

Changes include:
- Use `isTemplated()` instead of `isDependentContext()` since the latter doesn't
  necessarily handle templated friend functions.

Changes include:
- Replaced a use of `llvm::report_fatal_error()` with `assert()`.
- Modified `registerSYCLEntryPointFunction()` to silently skip registration
  of invalid or dependent function declarations.

Changes include:
- Minor refactoring to eliminate a branch that only contained an assertion.