The frontend option '-clang-header-expose-module' allows the user to specify that APIs from an imported module have been exposed in another generated header, and thus APIs that depend on them can be safely exposed in the current generated header.
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
