[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).

The original version of this patch was reverted in r309995 due to a
crash. This version includes a fix for that crash (testcase in 
test/CoverageMapping/md.cpp).

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

llvm-svn: 310406