[Clang] Avoid using unwind library in the MSVC environment

[phosek]

Summary

We're seeing the following warnings with --rtlib=compiler-rt:

  
lld-link: warning: ignoring unknown argument '--as-needed'
lld-link: warning: ignoring unknown argument '-lunwind'
lld-link: warning: ignoring unknown argument '--no-as-needed'

MSVC doesn't use the unwind library, so just omit it.

[mstorsjo]

LGTM.

(Should we have a testcase for this, or is it trivial enough to just go as is?)

[phosek]

Test added.

[mstorsjo]

This reverts commit eca29d4a37b8d1c93fe99be6289a60bb11cf789d since the test fails in the per-target-runtime-dir layout.

Does that mean that the testcase ended up looking at other libraries next to clang, in the build dir where the tests are run? That's rather annoying... I guess it could be fixed by adding -resource-dir in the testcase? (And ideally in the future, by making a harder switch between the two runtime install layouts in clang, as discussed on Discourse.)

[phosek]

In D132440#3744960, @mstorsjo wrote:

This reverts commit eca29d4a37b8d1c93fe99be6289a60bb11cf789d since the test fails in the per-target-runtime-dir layout.

Does that mean that the testcase ended up looking at other libraries next to clang, in the build dir where the tests are run? That's rather annoying... I guess it could be fixed by adding -resource-dir in the testcase? (And ideally in the future, by making a harder switch between the two runtime install layouts in clang, as discussed on Discourse.)

Yes, that's the issue, I added -resource-dir which is the solution we use in other tests as well.

clang/lib/Driver/ToolChains/CommonArgs.cpp

 // Also, certain targets need additional adjustments.
 
 static void AddUnwindLibrary(const ToolChain &TC, const Driver &D,
                              ArgStringList &CmdArgs, const ArgList &Args) {
   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)
+      TC.getTriple().isWindowsMSVCEnvironment() || UNW == ToolChain::UNW_None)
     return;
 
   LibGccType LGT = getLibGccType(TC, D, Args);
   bool AsNeeded = LGT == LibGccType::UnspecifiedLibGcc &&
                   (UNW == ToolChain::UNW_CompilerRT || !D.CCCIsCXX()) &&
                   !TC.getTriple().isAndroid() &&
                   !TC.getTriple().isOSCygMing() && !TC.getTriple().isOSAIX();
   if (AsNeeded)

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

 //
 // RUN: not %clang %s 2> %t.err              \
 // RUN:     --target=x86_64-unknown-linux -rtlib=libgcc --unwindlib=libunwind \
 // RUN:     --gcc-toolchain="" -resource-dir=%S/Inputs/resource_dir \
 // RUN: FileCheck --input-file=%t.err --check-prefix=RTLIB-GCC-UNWINDLIB-COMPILER_RT %s
 // RTLIB-GCC-UNWINDLIB-COMPILER_RT: "{{[.|\\\n]*}}--rtlib=libgcc requires --unwindlib=libgcc"
 //
 // RUN: %clang -### %s 2>&1 \
+// RUN:     --target=x86_64-pc-windows-msvc -rtlib=compiler-rt --unwindlib=libunwind \
+// RUN:   | FileCheck --check-prefix=MSVC-RTLIB-COMPILER-RT-UNWINDLIB-COMPILER-RT %s
+// MSVC-RTLIB-COMPILER-RT-UNWINDLIB-COMPILER-RT: "{{.*}}clang_rt.builtins-x86_64.lib"
+// MSVC-RTLIB-COMPILER-RT-UNWINDLIB-COMPILER-RT-NOT: "{{.*}}unwind.lib"
+//
+// RUN: %clang -### %s 2>&1 \
 // RUN:     --target=x86_64-w64-mingw32 -rtlib=compiler-rt --unwindlib=libunwind \
 // RUN:     -shared-libgcc \
 // RUN:     --gcc-toolchain="" \
 // RUN:   | FileCheck --check-prefix=MINGW-RTLIB-COMPILER-RT-SHARED-UNWINDLIB-COMPILER-RT %s
 // MINGW-RTLIB-COMPILER-RT-SHARED-UNWINDLIB-COMPILER-RT: "{{.*}}libclang_rt.builtins-x86_64.a"
 // MINGW-RTLIB-COMPILER-RT-SHARED-UNWINDLIB-COMPILER-RT-SAME: "-l:libunwind.dll.a"
 //
 // RUN: %clang -### %s 2>&1 \