[Driver] Always use --as-needed with libunwind

[phosek]

Summary

With libgcc, we follow the behavior of GCC for backwards compatibility,
only using --as-needed in the non-C++ mode.

With libunwind, there are no backward compatibility requirements so we
can always use --as-needed on all supported platforms.

clang/lib/Driver/ToolChains/CommonArgs.cpp

static void AddUnwindLibrary(const ToolChain &TC, const Driver &D,
   ToolChain::UnwindLibType UNW = TC.GetUnwindLibType(Args);
   // Targets that don't use unwind libraries.
   if ((TC.getTriple().isAndroid() && UNW == ToolChain::UNW_Libgcc) ||
       TC.getTriple().isOSIAMCU() || TC.getTriple().isOSBinFormatWasm() ||
       UNW == ToolChain::UNW_None)
     return;
 
   LibGccType LGT = getLibGccType(TC, D, Args);
-  bool AsNeeded = LGT == LibGccType::UnspecifiedLibGcc && !D.CCCIsCXX() &&
+  bool AsNeeded = LGT == LibGccType::UnspecifiedLibGcc &&
+                  (UNW == ToolChain::UNW_CompilerRT || !D.CCCIsCXX()) &&
                   !TC.getTriple().isAndroid() &&
                   !TC.getTriple().isOSCygMing() && !TC.getTriple().isOSAIX();
   if (AsNeeded)
     CmdArgs.push_back(getAsNeededOption(TC, true));
 
   switch (UNW) {
   case ToolChain::UNW_None:
     return;

clang/test/Driver/compiler-rt-unwind.c

 // General tests that the driver handles combinations of --rtlib=XXX and
 // --unwindlib=XXX properly.
 //
 // RUN: %clang -### %s 2>&1 \
 // RUN:     --target=x86_64-unknown-linux -rtlib=libgcc --unwindlib=platform \
 // RUN:     --gcc-toolchain="" -resource-dir=%S/Inputs/resource_dir \
 // RUN:   | FileCheck --check-prefix=RTLIB-GCC %s
-// RTLIB-GCC: "{{.*}}lgcc"
-// RTLIB-GCC: "{{.*}}lgcc_s"
+// RTLIB-GCC: "-lgcc"
+// RTLIB-GCC-SAME: "--as-needed"
+// RTLIB-GCC-SAME: "-lgcc_s"
+// RTLIB-GCC-SAME: "--no-as-needed"
+//
+// RUN: %clangxx -### %s 2>&1 \
+// RUN:     --target=x86_64-unknown-linux -rtlib=libgcc --unwindlib=platform \
+// RUN:     --gcc-toolchain="" -resource-dir=%S/Inputs/resource_dir \
+// RUN:   | FileCheck --check-prefix=RTLIB-GXX %s
+// RTLIB-GXX: "-lgcc"
+// RTLIB-GXX-NOT: "--as-needed"
+// RTLIB-GXX-SAME: "-lgcc_s"
+// RTLIB-GXX-NOT: "--no-as-needed"
 //
 // RUN: %clang -### %s 2>&1 \
 // RUN:     --target=x86_64-unknown-linux -rtlib=libgcc --unwindlib=libunwind \
 // RUN:     --gcc-toolchain="" -resource-dir=%S/Inputs/resource_dir \
 // RUN:   | FileCheck --check-prefix=RTLIB-GCC-UNWINDLIB-COMPILER-RT %s
-// RTLIB-GCC-UNWINDLIB-COMPILER-RT: "{{.*}}lgcc"
-// RTLIB-GCC-UNWINDLIB-COMPILER-RT: "{{.*}}lunwind"
+// RUN: %clangxx -### %s 2>&1 \
+// RUN:     --target=x86_64-unknown-linux -rtlib=libgcc --unwindlib=libunwind \
+// RUN:     --gcc-toolchain="" -resource-dir=%S/Inputs/resource_dir \
+// RUN:   | FileCheck --check-prefix=RTLIB-GCC-UNWINDLIB-COMPILER-RT %s
+// RTLIB-GCC-UNWINDLIB-COMPILER-RT: "-lgcc"
+// RTLIB-GCC-UNWINDLIB-COMPILER-RT-SAME: "--as-needed"
+// RTLIB-GCC-UNWINDLIB-COMPILER-RT-SAME: "-lunwind"
+// RTLIB-GCC-UNWINDLIB-COMPILER-RT-SAME: "--no-as-needed"
 //
 // RUN: %clang -### %s 2>&1 \
 // RUN:     --target=x86_64-unknown-linux -rtlib=libgcc --unwindlib=libunwind \
 // RUN:     -shared-libgcc \
 // RUN:     --gcc-toolchain="" -resource-dir=%S/Inputs/resource_dir \
 // RUN:   | FileCheck --check-prefix=RTLIB-GCC-SHARED-UNWINDLIB-COMPILER-RT %s
 // RTLIB-GCC-SHARED-UNWINDLIB-COMPILER-RT: "{{.*}}l:libunwind.so"
 // RTLIB-GCC-SHARED-UNWINDLIB-COMPILER-RT: "{{.*}}lgcc"
 //
 // RUN: %clang -### %s 2>&1 \
 // RUN:     --target=x86_64-unknown-linux -rtlib=libgcc --unwindlib=libunwind \
 // RUN:     -static-libgcc \
 // RUN:     --gcc-toolchain="" -resource-dir=%S/Inputs/resource_dir \
 // RUN:   | FileCheck --check-prefix=RTLIB-GCC-STATIC-UNWINDLIB-COMPILER-RT %s
 // RTLIB-GCC-STATIC-UNWINDLIB-COMPILER-RT: "{{.*}}lgcc"
 // RTLIB-GCC-STATIC-UNWINDLIB-COMPILER-RT: "{{.*}}l:libunwind.a"
 //
 // RUN: %clang -### %s 2>&1   \
 // RUN:     --target=x86_64-unknown-linux -rtlib=compiler-rt \
 // RUN:     --gcc-toolchain="" -resource-dir=%S/Inputs/resource_dir \
 // RUN:   | FileCheck --check-prefix=RTLIB-COMPILER-RT %s
 // RTLIB-COMPILER-RT: "{{.*}}libclang_rt.builtins-x86_64.a"
 //
-// RUN: %clang -### %s 2>&1   \
+// RUN: %clang -### %s 2>&1 \
+// RUN:     --target=x86_64-unknown-linux -rtlib=compiler-rt --unwindlib=libunwind \
+// RUN:     --gcc-toolchain="" -resource-dir=%S/Inputs/resource_dir \
+// RUN:   | FileCheck --check-prefix=RTLIB-COMPILER-RT-UNWINDLIB-COMPILER-RT %s
+// RUN: %clangxx -### %s 2>&1 \
+// RUN:     --target=x86_64-unknown-linux -rtlib=compiler-rt --unwindlib=libunwind \
+// RUN:     --gcc-toolchain="" -resource-dir=%S/Inputs/resource_dir \
+// RUN:   | FileCheck --check-prefix=RTLIB-COMPILER-RT-UNWINDLIB-COMPILER-RT %s
+// RTLIB-COMPILER-RT-UNWINDLIB-COMPILER-RT: "{{.*}}libclang_rt.builtins-x86_64.a"
+// RTLIB-COMPILER-RT-UNWINDLIB-COMPILER-RT-SAME: "--as-needed"
+// RTLIB-COMPILER-RT-UNWINDLIB-COMPILER-RT-SAME: "-lunwind"
+// RTLIB-COMPILER-RT-UNWINDLIB-COMPILER-RT-SAME: "--no-as-needed"
+//
+// RUN: %clang -### %s 2>&1   \
 // RUN:     --target=x86_64-unknown-linux -rtlib=compiler-rt --unwindlib=libgcc \
 // RUN:     --gcc-toolchain="" -resource-dir=%S/Inputs/resource_dir \
 // RUN:   | FileCheck --check-prefix=RTLIB-COMPILER-RT-UNWINDLIB-GCC %s
 // RTLIB-COMPILER-RT-UNWINDLIB-GCC: "{{.*}}libclang_rt.builtins-x86_64.a"
 // RTLIB-COMPILER-RT-UNWINDLIB-GCC: "{{.*}}lgcc_s"
 //
 // RUN: %clang -### %s 2>&1              \
 // RUN:     --target=x86_64-unknown-linux -rtlib=compiler-rt --unwindlib=libgcc \