Decls with @_spi_available aren't hidden entirely from public interfaces,
thus public interfaces may still refer to them. Be forgiving for such cases so public
interfaces can compile.
rdar://90938725
For clang symbols marked with SPI_AVAILABLE, we add SPIAccessControlAttr to them so they will be
considered as SPIs in the AST. To be able to use all these symbols, we also add an implicit SPI import
statement for all clang modules. All clang SPIs belong to the same SPI group named "OBJC_DEFUALT_SPI_GROUP" because clang
currently doesn't support custom SPI group.
rdar://73902734
