If the C++ compiler doesn't support the transparent_stepping attribute,
fallback to annotating inline thunks with the alwaysinline and nodebug
attributes.
Use transparent_stepping instead of nodebug and alwaysinline in compiler
generated C++ code from Swift. This attribute allow for annotated
functions to be used in expression evaluation, which is an advantage
over the former two.
This macro applies always_inline in addition to inline. It also applies artificial, which lets debugger know that this is an artificial function. The used attribute is added in debug builds to ensure that the symbol is emitted in the binary so that LLDB can invoke it.
Each emitted declaration is annotated with the external_source_symbol with its own USR, to allow Clang's indexer to recognize this declaration as a Swift declaration with a specific USR
The _SwiftCxxInteroperability.h is too tightly coupled to the compiler right now and should not be shipped with the standard library. In the future it could be moved back to the standard library.
