Driver: introduce -sysroot option for non-Darwin targets

This introduces a secondary flag `-sysroot` for the non-Darwin targets,
primarily Unicies. The intention here is to support a split `-sdk`,
`-sysroot` model where the `-sdk` parameter provides the Swift "SDK"
which augments the native platform's C sysroot which is indicated as
`-sysroot`. For the case of Android, this would allow us to provide a
path to the NDK sysroot and the Swift SDK allowing us to cross-compile
Android binaries from Windows.

Author: compnerd

include/swift/AST/SearchPathOptions.h

@@ -368,6 +368,8 @@ class SearchPathOptions {
   std::optional<StringRef> VCToolsRoot = std::nullopt;
   std::optional<StringRef> VCToolsVersion = std::nullopt;
 
+  std::optional<StringRef> SysRoot = std::nullopt;
+
 public:
   StringRef getSDKPath() const { return SDKPath; }
 
@@ -406,6 +408,11 @@ class SearchPathOptions {
     VCToolsVersion = version;
   }
 
+  std::optional<StringRef> getSysRoot() const { return SysRoot; }
+  void setSysRoot(StringRef sysroot) {
+    SysRoot = sysroot;
+  }
+
   ArrayRef<std::string> getImportSearchPaths() const {
     return ImportSearchPaths;
   }

include/swift/Option/Options.td

@@ -258,6 +258,13 @@ def visualc_tools_version : Separate<["-"], "visualc-tools-version">,
          SwiftSymbolGraphExtractOption, SwiftAPIDigesterOption]>,
   HelpText<"VisualC++ ToolSet Version">, MetaVarName<"<version>">;
 
+// Android Options
+
+def sysroot : Separate<["-"], "sysroot">,
+  Flags<[ArgumentIsPath, FrontendOption, SwiftAPIExtractOption,
+         SwiftSymbolGraphExtractOption, SwiftAPIDigesterOption]>,
+  HelpText<"Native Platform sysroot">, MetaVarName<"<sysroot>">;
+
 // Standard Options
 def _DASH_DASH : Option<["--"], "", KIND_REMAINING_ARGS>,
   Flags<[FrontendOption, DoesNotAffectIncrementalBuild]>;

lib/ClangImporter/ClangIncludePaths.cpp

@@ -175,6 +175,8 @@ createClangArgs(const ASTContext &ctx, clang::driver::Driver &clangDriver) {
   auto sdkPath = ctx.SearchPathOpts.getSDKPath();
   if (!sdkPath.empty())
     clangDriver.SysRoot = sdkPath.str();
+  if (auto sysroot = ctx.SearchPathOpts.getSysRoot())
+    clangDriver.SysRoot = sysroot->str();
   return clangDriverArgs;
 }
 

lib/Driver/ToolChains.cpp

@@ -243,6 +243,10 @@ void ToolChain::addCommonFrontendArgs(const OutputInfo &OI,
     arguments.push_back(inputArgs.MakeArgString(A->getValue()));
   }
 
+  if (const Arg *A = inputArgs.getLastArg(options::OPT_sysroot)) {
+    arguments.push_back("-sysroot");
+    arguments.push_back(inputArgs.MakeArgString(A->getValue()));
+  }
 
   if (llvm::sys::Process::StandardErrHasColors()) {
     arguments.push_back("-color-diagnostics");

lib/Frontend/CompilerInvocation.cpp

@@ -1919,6 +1919,9 @@ static bool ParseSearchPathArgs(SearchPathOptions &Opts,
   if (const Arg *A = Args.getLastArg(OPT_visualc_tools_version))
     Opts.setVCToolsVersion(A->getValue());
 
+  if (const Arg *A = Args.getLastArg(OPT_sysroot))
+    Opts.setSysRoot(A->getValue());
+
   if (const Arg *A = Args.getLastArg(OPT_resource_dir))
     Opts.RuntimeResourcePath = A->getValue();