ClangImporter attributes should follow the Swift attribute grammar

[susmonteiro]

The SWIFT_COPYABLE_IF macro expands to

#define SWIFT_COPYABLE_IF(...) \
  __attribute__((swift_attr("copyable_if:" _CXX_INTEROP_CONCAT(__VA_ARGS__))))

which requires a custom parser. If it followed the Swift attribute grammar, e.g. swift_attr("Copyable(if: " _CXX_INTEROP_CONCAT(__VA_ARGS__) ")" ), the standard Swift attribute parser would be able to parse it.

We should rewrite these existing attributes to follow the Swift attribute grammar, which will reduce complexity in the ClangImporter.