This patch addresses an ABI compatibility issue with empty aggregate

This patch addresses an ABI compatibility issue with empty aggregate
parameters.  Examples of these are:

  struct { } a;
  union { } b[256];
  int a[0];

An empty aggregate has an address, although dereferencing that address is
pointless.  When passed as a parameter, an empty aggregate does not consume
a protocol register, nor does it consume a doubleword in the parameter save
area.  Passing an empty aggregate by reference passes an address just as
for any other aggregate.  Returning an empty aggregate uses GPR3 as a hidden
address of the return value location, just as for any other aggregate.

The patch modifies PPCTargetLowering::LowerFormalArguments_64SVR4 and
PPCTargetLowering::LowerCall_64SVR4 to properly skip empty aggregate
parameters passed by value.  The handling of return values and by-reference
parameters was already correct.

Built on powerpc64-unknown-linux-gnu and tested with no new regressions.
A test case is included to test proper handling of empty aggregate
parameters on both sides of the function call protocol.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167090 91177308-0d34-0410-b5e6-96231b3b80d8