First pass at abstracting out a class for the target C++ ABI.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173514 91177308-0d34-0410-b5e6-96231b3b80d8
The standard ARM C++ ABI dictates that inline functions are
never key functions.  We did not implement that rule for the
iOS ABI, which was driven by what was implemented in gcc-4.2.
However, implement it now for other ARM-based platforms.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173515 91177308-0d34-0410-b5e6-96231b3b80d8