CallingConvLower.h: Use bitfields like unsigned:1 instead of bool:1 .

Some compilers might be confused if bool were potentially signed integer. In my case, g++-4.7.0 miscompiled CodeGen/ARM.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171727 91177308-0d34-0410-b5e6-96231b3b80d8