[MSVC] Turn C++ EH on my default

Our support for C++ EH is sufficiently good that it makes sense to
enable support for it out of the box.

While we are here, update the MSVCCompatibility doc.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@261195 91177308-0d34-0410-b5e6-96231b3b80d8