ExceptionHandling documentation tweaks.

ExceptionHandling documentation tweaks.

Delete mention of the llvm.eh.begincatch/llvm.eh.endcatch intrinsics,
and remove them from a few remaining test-cases. These intrinsics were
from a previous attempt at implementing Windows exception-handling,
but were removed from LLVM in 2015.

Also mention that dynamic exception specifications ("throw filters")
were removed from the spec in C++17.