[Driver] -fno-plt: warn for unsupported targets

-fno-plt is an ELF specific option that is only implemented for x86 (for
a long time) and AArch64 (#78890). GCC doesn't bother to give a
diagnostic on Windows. -fno-plt is somewhat popular and we've been
ignoring it for unsupported targets for a while, so just report a
warning for unsupported targets.

Pull Request: https://github.com/llvm/llvm-project/pull/124081

clang/lib/Driver/ToolChains/Clang.cpp

diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 33f08cf..518113e 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -6141,9 +6141,8 @@
     CmdArgs.push_back("-fno-direct-access-external-data");
   }
 
-  if (Args.hasFlag(options::OPT_fno_plt, options::OPT_fplt, false)) {
-    CmdArgs.push_back("-fno-plt");
-  }
+  if (Triple.isOSBinFormatELF() && (Triple.isAArch64() || Triple.isX86()))
+    Args.addOptOutFlag(CmdArgs, options::OPT_fplt, options::OPT_fno_plt);
 
   // -fhosted is default.
   // TODO: Audit uses of KernelOrKext and see where it'd be more appropriate to

clang/test/Driver/fno-plt.c

diff --git a/clang/test/Driver/fno-plt.c b/clang/test/Driver/fno-plt.c
new file mode 100644
index 0000000..c7bd713
--- /dev/null
+++ b/clang/test/Driver/fno-plt.c
@@ -0,0 +1,10 @@
+// RUN: %clang -### -c --target=aarch64 -fno-plt -Werror %s 2>&1 | FileCheck %s --check-prefix=NOPLT
+// RUN: %clang -### -c --target=x86_64 -fno-plt -Werror %s 2>&1 | FileCheck %s --check-prefix=NOPLT
+
+// RUN: %clang -### -c --target=aarch64 -fno-plt -fplt -Werror %s 2>&1 | FileCheck %s --check-prefix=DEFAULT
+// RUN: %clang -### -c --target=powerpc64 -fno-plt %s 2>&1 | FileCheck %s --check-prefixes=WARN,DEFAULT
+// RUN: %clang -### -c --target=aarch64-windows -fno-plt %s 2>&1 | FileCheck %s --check-prefixes=WARN,DEFAULT
+
+// WARN: warning: argument unused during compilation: '-fno-plt' [-Wunused-command-line-argument]
+// NOPLT: "-fno-plt"
+// DEFAULT-NOT: "-fno-plt"