ARM: Proactively ensure that the LowerCallResult hack for 'this'-returns is not used for incompatible calling conventions.

(Currently, ARM 'this'-returns are handled in the standard calling convention case by treating R0 as preserved and doing some extra magic in LowerCallResult; this may not apply to calling conventions added in the future so this patch provides and documents an interface for indicating such)


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185024 91177308-0d34-0410-b5e6-96231b3b80d8