[ASTGen] Separate JSON serialization module from ASTGen
For future usage from other host libraries written in Swift
For CMake:
* Explicitly specify LINKER_LANGAGE to CXX in existing components so
that 'swiftc' is not used when linking with 'swiftASTGen'
* Add 'EMIT_MODULE' argument to 'add_pure_swift_host_library' to emit
.swiftmodule usable from other Swift libraries.
