[LLD][ELF]Add test for missing thin archive member

[jhenderson]

Summary

The code producing error messages relating to missing thin archive members was missing any testing as far as I could see, so this patch adds a test for it.

[ruiu]

LGTM

test/ELF/archive-thin-missing-member.s

+# REQUIRES: x86
+
+# RUN: rm -f %t-no-syms.a
+# RUN: rm -f %t-syms.a
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: llvm-ar rcTS %t-no-syms.a %t.o
+# RUN: llvm-ar rcT %t-syms.a %t.o
+# RUN: rm %t.o
+
+# Test error when loading symbols from missing thin archive member.
+# RUN: not ld.lld %t-no-syms.a 2>&1 | FileCheck %s --check-prefix=ERR1
+# ERR1: {{.*}}-no-syms.a: could not get the buffer for a child of the archive: '{{.*}}.o': {{[Nn]}}o such file or directory
+
+# Test error when thin archive has symbol table but member is missing.
+# RUN: not ld.lld -m elf_amd64_fbsd %t-syms.a 2>&1 | FileCheck %s --check-prefix=ERR2
+# ERR2: {{.*}}-syms.a: could not get the buffer for the member defining symbol _start: '{{.*}}.o': {{[Nn]}}o such file or directory
+
+# Test error when thin archive is linked using --whole-archive but member is missing.
+# RUN: not ld.lld --whole-archive %t-syms.a 2>&1 | FileCheck %s --check-prefix=ERR3
+# ERR3: {{.*}}-syms.a: could not get the buffer for a child of the archive: '{{.*}}.o': {{[Nn]}}o such file or directory
+
+.global _start
+_start:
+    nop