[SYCL] Enable let PragmaAttributeSupport for SYCL attributes

[elizabethandrews]

The typical upstream expectation is that new attributes are supported via #pragma clang attribute (https://clang.llvm.org/docs/LanguageExtensions.html#specifying-an-attribute-for-multiple-declarations-pragma-clang-attribute) unless there's some reason for them not to (like statement or type attributes).

This does NOT enable pragma spellings for attribute as previously thought and is actually a useful functionality. See comment thread - #2877 (comment) for details