swift234

Add frontend option "-warn-long-function-bodies=<N>".

This is a /slightly/ more user-friendly option than
-debug-time-function-bodies; pass it a limit in milliseconds and
the compiler will warn whenever a function or multi-statement closure
takes longer than that to type-check.

Since it's a frontend option (and thus usually passed with -Xfrontend),
I went with the "joined" syntax as the common case. The usual "separate"
syntax of "-warn-long-function-bodies <N>" is also available.

As a frontend option, this is UNSUPPORTED and may be removed without
notice at any future date.

Additional caveats:
- Other parts of type-checking not measured by this may also be slow.
- May include first-use penalties (i.e. "this is slow because it's
  the first function that references an imported type, which causes
  many things to be imported")
- Does not report anything whatsoever about other phases of compilation
  (SILGen, optimization, IRGen, assembly emission, whatever).
- Does not catch anything accidentally being type-checked multiple times
  (a known issue for initial value expressions on properties).