Name binding can trigger swiftinterface compilation, which creates
a new ASTContext and runs a compilation job. If the compiler was
run with -stats-output-dir, this could trigger an assertion because
SharedTimer is not re-entrant.
Fix this by replacing all direct uses of SharedTimer in the frontend
with FrontendStatsTracer. SharedTimer is still used to _implement_
FrontendStatsTracer, however we can collapse some of the layers in
the implementation later. Many of the usages should also become
redundant over time once more code is converted over to requests.
