[WinEH] Update exception pointer registers

[WinEH] Update exception pointer registers

Summary:
The CLR's personality routine passes these in rdx/edx, not rax/eax.

Make getExceptionPointerRegister a virtual method parameterized by
personality function to allow making this distinction.

Similarly make getExceptionSelectorRegister a virtual method parameterized
by personality function, for symmetry.

Reviewers: pgavlin, majnemer, rnk

Subscribers: jyknight, dsanders, llvm-commits

Differential Revision: http://reviews.llvm.org/D14344

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252383 91177308-0d34-0410-b5e6-96231b3b80d8