[WinEH] Add llvm.eh.exceptionpointer intrinsic
Summary:
This intrinsic can be used to extract a pointer to the exception caught by
a given catchpad. Its argument has token type and must be a `catchpad`.
Also clarify ExtendingLLVM documentation regarding overloaded intrinsics.
Reviewers: majnemer, andrew.w.kaylor, sanjoy, rnk
Subscribers: llvm-commits
Differential Revision: http://reviews.llvm.org/D12533
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246752
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