Use custom ABIInfo for le32/PNaCl argument codegen

This patch uses a new ABIInfo implementation specific to the le32
target, rather than falling back to DefaultABIInfo. Its behavior is
basically the same, but it also allows the regparm argument attribute.
It also includes basic tests for argument codegen and attributes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@163333 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed