This patch addresses an incompatibility relative to the 64-bit PowerPC

This patch addresses an incompatibility relative to the 64-bit PowerPC
ELF ABI.

Complex values are to be passed in registers as though the real and
imaginary parts were passed as separate parameters.  Prior to this
patch, complex values were passed as byval aggregates.  It turns out
that specifying getDirect() for all complex types when classifying the
argument type results in the desired behavior.

The new Clang test case verifies that the correct LLVM IR is generated
for caller and callee for each of the underlying types for _Complex.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@168673 91177308-0d34-0410-b5e6-96231b3b80d8