[TLI] Add functions determining if int parameters/returns should be zeroext/signext.

On some architectures (s390x, ppc64, sparc64, mips), C-level int is passed
as i32 signext instead of plain i32.  Likewise, unsigned int may be passed
as i32, i32 signext, or i32 zeroext depending on the platform.  Add this
information to TargetLibraryInfo, to be used whenever some LLVM pass
inserts a compiler-rt call to a function involving int parameters
or returns.

Differential Revision: http://reviews.llvm.org/D21739

llvm-svn: 287533