Warn when instrumenting a Module with unhandled external Functions.

Add X-Macro system include/seec/Transforms/FunctionsHandled.def which lists all
handled functions (either detect-calls handled or intercepted).

Keep a set of all unhandled external llvm::Function * on the
InsertExternalRecording pass.

In seec-instrument, print a warning for each unhandled external llvm::Function.