[WebAssembly] Add an llvm-lto path for compiler-rt.

[sunfish]

Summary

This allows LTO-enabled builds of compiler-rt to be used.

[sunfish]

I've done some more experimenting with this, and it turns out not to work very well, because calls to these functions get generated after LTO runs, so I'll abandon this.

clang/lib/Driver/ToolChains/WebAssembly.h

void AddClangCXXStdlibIncludeArgs(
       llvm::opt::ArgStringList &CC1Args) const override;
   void AddCXXStdlibLibArgs(const llvm::opt::ArgList &Args,
                            llvm::opt::ArgStringList &CmdArgs) const override;
   SanitizerMask getSupportedSanitizers() const override;
 
   const char *getDefaultLinker() const override { return "wasm-ld"; }
 
   Tool *buildLinker() const override;
+  std::string getCompilerRTPath() const;
 };
 
 } // end namespace toolchains
 } // end namespace driver
 } // end namespace clang
 
 #endif // LLVM_CLANG_LIB_DRIVER_TOOLCHAINS_WEBASSEMBLY_H

clang/lib/Driver/ToolChains/WebAssembly.cpp

if (D.isUsingLTO()) {
       // bitcode format is not stable.
       auto Dir = AppendLTOLibDir(SysRoot + "/lib/" + MultiarchTriple);
       getFilePaths().push_back(Dir);
     }
     getFilePaths().push_back(SysRoot + "/lib/" + MultiarchTriple);
   }
 }
 
+std::string WebAssembly::getCompilerRTPath() const {
+  // If we're doing LTO and we have an LTO library available, use it.
+  const auto &D = getDriver();
+  if (D.isUsingLTO()) {
+    auto Dir = AppendLTOLibDir(ToolChain::getCompilerRTPath());
+    if (llvm::sys::fs::exists(Dir))
+      return Dir;
+  }
+
+  // Otherwise just use the default.
+  return ToolChain::getCompilerRTPath();
+}
+
 bool WebAssembly::IsMathErrnoDefault() const { return false; }
 
 bool WebAssembly::IsObjCNonFragileABIDefault() const { return true; }
 
 bool WebAssembly::UseObjCMixedDispatch() const { return true; }
 
 bool WebAssembly::isPICDefault() const { return false; }