[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 91177308-0d34-0410-b5e6-96231b3b80d8