[wasm][Driver] HACK: Add wasi-sysroot layout and auto-detection support

Currenty, SwiftWasm toolchain contains wasi-sysroot under
`./usr/share/wasi-sysroot` directory in the toolchain package, but we
don't have a good consensus on the location of wasi-sysroot yet. So this
patch is a temporary solution just to avoid the need to specify `-sdk`
option manually. That being said, SwiftPM already passes `-sdk` option
to the driver, so the option is only needed when the driver is invoked
directly.

Author: kateinoigakukun

lib/Driver/Driver.cpp

@@ -1795,6 +1795,13 @@ void Driver::buildOutputInfo(const ToolChain &TC, const DerivedArgList &Args,
           });
         }
       }
+    } if (OI.SDKPath.empty() && TC.getTriple().isOSWASI()) {
+        llvm::SmallString<128> SDKPath;
+        llvm::sys::path::append(SDKPath, getSwiftProgramPath());
+        llvm::sys::path::remove_filename(SDKPath); // 'swift'
+        llvm::sys::path::remove_filename(SDKPath); // 'bin'
+        llvm::sys::path::append(SDKPath, "share", "wasi-sysroot");
+        OI.SDKPath = SDKPath.str().str();
     }
 
     if (!OI.SDKPath.empty()) {

lib/Driver/ToolChains.cpp

@@ -1448,7 +1448,8 @@ void ToolChain::getResourceDirPath(SmallVectorImpl<char> &resourceDirPath,
   if (const Arg *A = args.getLastArg(options::OPT_resource_dir)) {
     StringRef value = A->getValue();
     resourceDirPath.append(value.begin(), value.end());
-  } else if (!getTriple().isOSDarwin() && args.hasArg(options::OPT_sdk)) {
+  } else if (!getTriple().isOSDarwin() && !getTriple().isOSWASI() && args.hasArg(options::OPT_sdk)) {
+    // for WASI, sdk option points to wasi-sysroot which doesn't have Swift toolchain
     StringRef value = args.getLastArg(options::OPT_sdk)->getValue();
     resourceDirPath.append(value.begin(), value.end());
     llvm::sys::path::append(resourceDirPath, "usr");