[llvm] [llvm-ar][Object] Use K_GNU instead of K_COFF for archives with no symtab. (PR #82898)

[Jacek Caban via llvm-commits]

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+## Create archives with no symtab in various formats and check that we can read them.
+
+# RUN: yaml2obj %s -o %t.o
+# RUN: rm -f %t.*.a
+
+# RUN: llvm-ar --format=gnu rcS %t.gnu.a %t.o
+# RUN: llvm-ar --format=coff rcS %t.coff.a %t.o
+# RUN: llvm-ar --format=darwin rcS %t.darwin.a %t.o
+# RUN: llvm-ar --format=bsd rcS %t.bsd.a %t.o
+# RUN: llvm-ar --format=bigarchive rcS %t.bigarchive.a %t.o
+
+# RUN: llvm-nm --print-armap %t.gnu.a | FileCheck %s
+# RUN: llvm-nm --print-armap %t.coff.a | FileCheck %s
+# RUN: llvm-nm --print-armap %t.darwin.a | FileCheck %s
+# RUN: llvm-nm --print-armap %t.bsd.a | FileCheck %s
+# RUN: llvm-nm --print-armap %t.bigarchive.a | FileCheck %s
+
+# CHECK-NOT: Archive map
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cjacek wrote:

The interesting part is that `llvm-nm` does not fail to read the archive, FileCheck is just an extra check that felt better than nothing.

https://github.com/llvm/llvm-project/pull/82898