Allow multiple .eh_frame sections per object file.

LLVM is experimenting with placing .eh_frame sections in the COMDAT group
with the function's text. This triggers an internal error in gold because
we don't expect to see but one .eh_frame section in an object, and we use
a single data member in class Sized_relobj_file to keep track of that section.

This patch removes that data member, and instead checks the output section
and the input section offset to identify an optimized .eh_frame section.

gold/
	* object.h (class Sized_relobj_file): Remove discarded_eh_frame_shndx_.
	* object.cc (Sized_relobj_file::Sized_relobj_file): Likewise.
	(Sized_relobj_file::layout_eh_frame_section): Likewise.
	(Sized_relobj_file::do_count_local_symbols): Check for optimized
	.eh_frame section by other means.
	(Sized_relobj_file::compute_final_local_value_internal): Likewise.