RegionInfo: Do not crash if unreachable block is found

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181025 91177308-0d34-0410-b5e6-96231b3b80d8
Autoconf: Compile cxxabi.h in C++ mode.

Should fix PR15877.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181026 91177308-0d34-0410-b5e6-96231b3b80d8