[coverage] Special-case calls to noreturn functions.

The code after a noreturn call doesn't execute.

The pattern in the testcase is pretty common in LLVM (a switch with
a default case that calls llvm_unreachable).

Differential Revision: https://reviews.llvm.org/D36250



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@309995 91177308-0d34-0410-b5e6-96231b3b80d8