[clang][ARM] Enable --print-supported-extensions for ARM (#66083)

```
$ ./bin/clang --target=arm-linux-gnueabihf --print-supported-extensions
<...>
All available -march extensions for ARM

        crc
        crypto
        sha2
        aes
        dotprod
<...>
```

This follows the format set by RISC-V and AArch64. As for AArch64, ARM
doesn't have versioned extensions like RISC-V does. So there is only 1
column, which contains the name.

Any extension without a "feature" is hidden as these cannot be used with
-march.

1 files changed, 23 insertions, 0 deletions

diff --git a/llvm/unittests/TargetParser/TargetParserTest.cpp b/llvm/unittests/TargetParser/TargetParserTest.cpp
index d551eb9..8223f9f 100644
--- a/llvm/unittests/TargetParser/TargetParserTest.cpp
+++ b/llvm/unittests/TargetParser/TargetParserTest.cpp
@@ -1010,6 +1010,29 @@ TEST(TargetParserTest, getARMCPUForArch) {
   }
 }
 
+TEST(TargetParserTest, ARMPrintSupportedExtensions) {
+  std::string expected = "All available -march extensions for ARM\n\n"
+                         "\tcrc\n\tcrypto\n\tsha2";
+
+  outs().flush();
+  testing::internal::CaptureStdout();
+  ARM::PrintSupportedExtensions();
+  outs().flush();
+  std::string captured = testing::internal::GetCapturedStdout();
+
+  // Check that the start of the output is as expected.
+  EXPECT_EQ(0ULL, captured.find(expected));
+
+  // Should not include "none" or "invalid".
+  EXPECT_EQ(std::string::npos, captured.find("none"));
+  EXPECT_EQ(std::string::npos, captured.find("invalid"));
+  // Should not include anything that lacks a feature name. Checking a few here
+  // but not all as if one is hidden correctly the rest should be.
+  EXPECT_EQ(std::string::npos, captured.find("simd"));
+  EXPECT_EQ(std::string::npos, captured.find("maverick"));
+  EXPECT_EQ(std::string::npos, captured.find("xscale"));
+}
+
 class AArch64CPUTestFixture
     : public ::testing::TestWithParam<
           ARMCPUTestParams<AArch64::ExtensionBitset>> {};