Emitting fine-grained module tracing files during full compilation should be sufficient for the use case in design. Other
swift-frontend invocations, such as emitting modules, doesn't need fully type-checked AST to perform the action, therefore
requesting/walking the fully typechecked AST to emit those tracing files may introduce performance overhead.
When lazy type checking is enabled, the generated AST to walk on may contain function bodies that
are not fully type-checked. Walking on these function bodies could expose latent bugs.
Resolves rdar://140818215
To simplify post-processing of these collected ObjC message traces, this PR teaches the compiler to atomically append the content of
ObjC message trace to a file location provided by the build environment, instead of emitting multiple files.
