[clang][driver][wasm] Fix libstdc++ target-dependent include dir

[tbaeder]

Summary

As an alternative to https://reviews.llvm.org/D119056 that addresses the issues raised with that patch and fixes the test case to succeed.

I think this does what Jonathan explained in https://reviews.llvm.org/D119056#3313911

[tbaeder]

@sbc100 Do you think we should add /backward as well? Gnu.cpp does that it seems: https://github.com/llvm/llvm-project/blob/main/clang/lib/Driver/ToolChains/Gnu.cpp#L2959-L2960

[sbc100]

In D120251#3336981, @tbaeder wrote:

@sbc100 Do you think we should add /backward as well? Gnu.cpp does that it seems: https://github.com/llvm/llvm-project/blob/main/clang/lib/Driver/ToolChains/Gnu.cpp#L2959-L2960

I don't see why not.

clang/lib/Driver/ToolChains/WebAssembly.cpp

if (MaxVersion.Major > 0)
       Version = MaxVersion.Text;
   }
 
   if (Version.empty())
     return;
 
   // First add the per-target include path if the OS is known.
   if (IsKnownOs) {
-    std::string TargetDir = LibPath + "/" + MultiarchTriple + "/c++/" + Version;
+    std::string TargetDir = LibPath + "/c++/" + Version + "/" + MultiarchTriple;
     addSystemInclude(DriverArgs, CC1Args, TargetDir);
   }
 
   // Second add the generic one.
   addSystemInclude(DriverArgs, CC1Args, LibPath + "/c++/" + Version);
+  // Third the backward one.
+  addSystemInclude(DriverArgs, CC1Args, LibPath + "/c++/" + Version + "/backward");
 }

clang/test/Driver/wasm-toolchain.cpp

 // COMPILE: "-internal-isystem" "[[SYSROOT:[^"]+]]/include"
 
 // RUN: %clangxx -### -no-canonical-prefixes -target wasm32-wasi --stdlib=libstdc++ %s 2>&1 \
 // RUN:     --sysroot=%S/Inputs/basic_linux_libstdcxx_libcxxv2_tree/usr \
 // RUN:   | FileCheck -check-prefix=COMPILE_STDCXX %s
 // COMPILE_STDCXX: clang{{.*}}" "-cc1"
 // COMPILE_STDCXX: "-resource-dir" "[[RESOURCE_DIR:[^"]*]]"
 // COMPILE_STDCXX: "-isysroot" "[[SYSROOT:[^"]+]]"
-// COMPILE_STDCXX: "-internal-isystem" "[[SYSROOT:[^"]+]]/include/wasm32-wasi/c++/4.8"
+// COMPILE_STDCXX: "-internal-isystem" "[[SYSROOT:[^"]+]]/include/c++/4.8/wasm32-wasi"
 // COMPILE_STDCXX: "-internal-isystem" "[[SYSROOT:[^"]+]]/include/c++/4.8"
+// COMPILE_STDCXX: "-internal-isystem" "[[SYSROOT:[^"]+]]/include/c++/4.8/backward"
 // COMPILE_STDCXX: "-internal-isystem" "[[RESOURCE_DIR]]{{(/|\\\\)}}include"
 // COMPILE_STDCXX: "-internal-isystem" "[[SYSROOT:[^"]+]]/include/wasm32-wasi"
 // COMPILE_STDCXX: "-internal-isystem" "[[SYSROOT:[^"]+]]/include"