Fix passing of struct with bitfields on x86-64

Fix passing of struct with bitfields on x86-64

Commit 4aa866af ("Fix AMD64 return value ABI in expression
evaluation") introduced a regression when calling a function with a
structure that contains bitfields.

Because the caller of amd64_has_unaligned_fields handles bitfields
already, it seemed to me that the simplest fix was to ignore bitfields
here.

gdb/ChangeLog
2019-04-24  Tom Tromey  <tromey@adacore.com>

	* amd64-tdep.c (amd64_has_unaligned_fields): Ignore bitfields.

gdb/testsuite/ChangeLog
2019-04-24  Tom Tromey  <tromey@adacore.com>

	* gdb.arch/amd64-eval.exp: Test bitfield return.
	* gdb.arch/amd64-eval.cc (struct Bitfields): New.
	(class Foo) <return_bitfields>: New method.
	(main): Call it.