CFI: Introduce -fsanitize=cfi-icall flag.

This flag causes the compiler to emit bit set entries for functions as well
as runtime bitset checks at indirect call sites. Depends on the new function
bitset mechanism.

Differential Revision: http://reviews.llvm.org/D11857

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@247238 91177308-0d34-0410-b5e6-96231b3b80d8