[AIX] identify big archive magic as archive.

[hubert.reinterpretcast]

LGTM; thanks!

llvm/lib/BinaryFormat/Magic.cpp

file_magic llvm::identify_magic(StringRef Magic) {
   case 'B':
     if (startswith(Magic, "BC\xC0\xDE"))
       return file_magic::bitcode;
     break;
   case '!':
     if (startswith(Magic, "!<arch>\n") || startswith(Magic, "!<thin>\n"))
       return file_magic::archive;
     break;
-
+  case '<':
+    if (startswith(Magic, "<bigaf>\n"))
+      return file_magic::archive;
+    break;
   case '\177':
     if (startswith(Magic, "\177ELF") && Magic.size() >= 18) {
       bool Data2MSB = Magic[5] == 2;
       unsigned high = Data2MSB ? 16 : 17;
       unsigned low = Data2MSB ? 17 : 16;
       if (Magic[high] == 0) {
         switch (Magic[low]) {
         default:

llvm/unittests/BinaryFormat/TestFileMagic.cpp

void SetUp() override {
     errs() << "Test Directory: " << TestDirectory << '\n';
     errs().flush();
   }
 
   void TearDown() override { ASSERT_NO_ERROR(fs::remove(TestDirectory.str())); }
 };
 
 const char archive[] = "!<arch>\x0A";
+const char big_archive[] = "<bigaf>\x0A";
 const char bitcode[] = "\xde\xc0\x17\x0b";
 const char coff_object[] = "\x00\x00......";
 const char coff_bigobj[] =
     "\x00\x00\xff\xff\x00\x02......"
     "\xc7\xa1\xba\xd1\xee\xba\xa9\x4b\xaf\x20\xfa\xf6\x6a\xa4\xdc\xb8";
 const char coff_import_library[] = "\x00\x00\xff\xff....";
 const char elf_relocatable[] = {0x7f, 'E', 'L', 'F', 1, 2, 1, 0, 0,
                                 0,    0,   0,   0,   0, 0, 0, 0, 1};
TEST_F(MagicTest, Magic) {
   struct type {
     const char *filename;
     const char *magic_str;
     size_t magic_str_len;
     file_magic magic;
   } types[] = {
 #define DEFINE(magic) {#magic, magic, sizeof(magic), file_magic::magic}
       DEFINE(archive),
+      {"big_archive", big_archive, sizeof(big_archive), file_magic::archive},
       DEFINE(bitcode),
       DEFINE(coff_object),
       {"coff_bigobj", coff_bigobj, sizeof(coff_bigobj),
        file_magic::coff_object},
       DEFINE(coff_import_library),
       DEFINE(elf_relocatable),
       DEFINE(goff_object),
       DEFINE(macho_universal_binary),