Support shf link order

[quic-seaswara]

No description provided.

[quic-areg]

This PR is quite difficult to review because it bundles several independent changes. The core SHF_LINK_ORDER functionality should be split from the additions to existing --trace options, at least.

The new lit %standalone_inputs lit substitution introduced exclusively for these tests feels awkward and unnecessary, and introduces a convention used nowhere else in the test suite. The assembler-with-cpp +#include asm_section_type_macros.h + %standalone_inputs chain is unnecessary. It seems like this was added because @progbits syntax is not accepted on arm. %progbits should be accepted by all targets and allow us to remove all of these new constructions.

Functionality-wise, eld seems to allow what lld and bfd reject:

cat > 1.s << \!
.section .text.foo,"ax",%progbits
.globl foo
foo:
  .word 0

.section .text.bar,"ax",%progbits
.globl bar
bar:
  .word 0

.section .meta.bar,"ao",%progbits,.text.bar
.globl meta_bar
meta_bar:
  .word 0xBBBB

.text
.globl _start
_start:
  .word foo
  .word meta_bar
!

llvm-mc -filetype=obj -triple=aarch64-linux 1.s -o 1.o
ld.eld --gc-sections -e _start 1.o -o e.out
ld.lld --gc-sections -e _start 1.o -o l.out
/usr/bin/aarch64-linux-gnu-ld --gc-sections -e _start 1.o -o g.out

-ld.eld --gc-sections -e _start 1.o -o e.out
ld.lld --gc-sections -e _start 1.o -o l.out
+ld.lld: error: 1.o:(.meta.bar): sh_link points to discarded section 1.o:(.text.bar)
/usr/bin/aarch64-linux-gnu-ld --gc-sections -e _start 1.o -o g.out
+`meta_bar' referenced in section `.text' of 1.o: defined in discarded section `.meta.bar' of 1.o

I have other comments, but this should do for now.

[quic-seaswara]

Thanks for the review!. I will create a seperate PR for tracing diagnostics. Will remove asm-helper, good to know %progbits is accepted by all targets.