The parser uses a try-catch in order to record when the client of an iterator
throws.  The exception then used to get rethrown via 'throw', which
unfortunately resulted in the original exception message object getting
overwritten.

This CL solves this as follows:
- add a clear_pending_message flag to TryCatchStatement (set to true in normal
  cases),
- set clear_pending_message to false for the TryCatchStatement used in iterator
  finalization
- change full-codegen, turbofan, and the interpreter to emit the ClearPendingMessage call
  only when the flag is set,
- replace 'throw' with '%ReThrow' in the iterator finalization code, thus
  reusing the (not-cleared) pending message

R=littledan@chromium.org, mstarzinger@chromium.org, yangguo@chromium.org
BUG=v8:4875
LOG=n

Review URL: https://codereview.chromium.org/1842953003

Cr-Commit-Position: refs/heads/master@{#35226}