PNaCl: Change exception info format to distinguish catch-all/cleanup clauses

The initial version of ExceptionInfoWriter.cpp encodes a landingpad's
"cleanup" clause the same as "catch i8* null" (a catch-all).

But it turns out we do want to distinguish these two: If an uncaught
exception occurs, we don't want the runtime to run C++ destructors
(cleanups), because this involves unwinding the stack to jump to
landingpads, which loses the context of where the uncaught exception
was thrown from, which is unhelpful for debugging.  (The C++ standard
says it's optional whether destructors are run when an uncaught
exception occurs.)

So, change the encoding as follows:

 * Use 0 as the ID for "cleanup" clauses.
 * Add 1 to the IDs of types and "catch" clauses.  (Adding 1 to both
   seems neater than just one of them.)
 * This means we can use 0 for the terminator of filter lists instead
   of -1, which seems a little neater.

This requires a corresponding change to eh_pnacl.cc in libsupc++.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3696
TEST=NaCl's EH tests with libsupc++ change applied

Review URL: https://codereview.chromium.org/69923008

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