[WebAssembly] Change the default linker for wasm32-wasip2

This commit changes the default linker in the WebAssembly toolchain for
the `wasm32-wasip2` target. This target is being added to the
WebAssembly/wasi-sdk and WebAssembly/wasi-libc projects to target the
Component Model by default, in contrast with the preexisting
`wasm32-wasi` target (in the process of being renamed to
`wasm32-wasip1`) which outputs a core WebAssembly module by default.

The `wasm-component-ld` project currently lives in my GitHub account at
https://github.com/alexcrichton/wasm-component-ld and isn't necessarily
"official" yet, but it's expected to continue to evolve as the
`wasm32-wasip2` target continues to shape up and evolve.

Author: alexcrichton

clang/lib/Driver/ToolChains/WebAssembly.cpp

@@ -221,6 +221,12 @@ WebAssembly::WebAssembly(const Driver &D, const llvm::Triple &Triple,
   }
 }
 
+const char *WebAssembly::getDefaultLinker() const {
+  if (getOS() == "wasip2")
+    return "wasm-component-ld";
+  return "wasm-ld";
+}
+
 bool WebAssembly::IsMathErrnoDefault() const { return false; }
 
 bool WebAssembly::IsObjCNonFragileABIDefault() const { return true; }

clang/lib/Driver/ToolChains/WebAssembly.h

@@ -67,7 +67,7 @@ class LLVM_LIBRARY_VISIBILITY WebAssembly final : public ToolChain {
                            llvm::opt::ArgStringList &CmdArgs) const override;
   SanitizerMask getSupportedSanitizers() const override;
 
-  const char *getDefaultLinker() const override { return "wasm-ld"; }
+  const char *getDefaultLinker() const override;
 
   CXXStdlibType GetDefaultCXXStdlibType() const override {
     return ToolChain::CST_Libcxx;

clang/test/Driver/wasm-toolchain.c

@@ -197,3 +197,11 @@
 // RUN: not %clang -### %s --target=wasm32-unknown-unknown --sysroot=%s/no-sysroot-there -fPIC -mno-mutable-globals %s 2>&1 \
 // RUN:   | FileCheck -check-prefix=PIC_NO_MUTABLE_GLOBALS %s
 // PIC_NO_MUTABLE_GLOBALS: error: invalid argument '-fPIC' not allowed with '-mno-mutable-globals'
+
+// Test that `wasm32-wasip2` invokes the `wasm-component-ld` linker by default
+// instead of `wasm-ld`.
+
+// RUN: %clang -### -O2 --target=wasm32-wasip2 %s --sysroot /foo 2>&1 \
+// RUN:   | FileCheck -check-prefix=LINK_WASIP2 %s
+// LINK_WASIP2: "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
+// LINK_WASIP2: wasm-component-ld{{.*}}" "-L/foo/lib/wasm32-wasip2" "crt1.o" "[[temp]]" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"