[RISCV] Implement RISCV ABI lowering

[RISCV] Implement RISCV ABI lowering

RISCVABIInfo is implemented in terms of XLen, supporting both RV32 and RV64.
Unfortunately we need to count argument registers in the frontend in order to
determine when to emit signext and zeroext attributes. Integer scalars are
extended according to their type up to 32-bits and then sign-extended to XLen
when passed in registers, but are anyext when passed on the stack. This patch
only implements the base integer (soft float) ABIs.

For more information on the RISC-V ABI, see [the ABI
doc](https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md),
my [golden model](https://github.com/lowRISC/riscv-calling-conv-model), and
the [LLVM RISC-V calling convention
patch](https://reviews.llvm.org/D39898#2d1595b4) (specifically the comment
documenting frontend expectations).

Differential Revision: https://reviews.llvm.org/D40023

llvm-svn: 322494