This patch addresses PR13948.

For 64-bit PowerPC SVR4, an aggregate containing only one
floating-point field (float, double, or long double) must be passed in
a register as though just that field were present.  This patch
addresses the issue during Clang code generation by specifying in the
ABIArgInfo for the argument that the underlying type is passed
directly in a register.  The included test case verifies flat and
nested structs for the three data types.

llvm-svn: 165816