[Driver] Don't validate arclite linking if the driver is only set up to get the swift-frontend invocation

If the driver is only set up to get the frontend invocation (e.g. from sourcekitd), don't validate that we can link against ARCLite because
a) we don't care about link-time when we are only interested in the frontend invocation
b) finding arclite might cause us to find clang using xcrun which in turn can cause sourcekitd to hang.

rdar://50659268

Author: ahoppen

include/swift/Driver/Driver.h

@@ -200,6 +200,11 @@ class Driver {
   /// Indicates whether the driver should check that the input files exist.
   bool CheckInputFilesExist = true;
 
+  /// Indicates that this driver never actually executes any commands but is
+  /// just set up to retrieve the swift-frontend invocation that would be
+  /// executed during compilation.
+  bool IsDummyDriverForFrontendInvocation = false;
+
 public:
   Driver(StringRef DriverExecutable, StringRef Name,
          ArrayRef<const char *> Args, DiagnosticEngine &Diags);
@@ -226,6 +231,14 @@ class Driver {
 
   void setCheckInputFilesExist(bool Value) { CheckInputFilesExist = Value; }
 
+  bool isDummyDriverForFrontendInvocation() const {
+    return IsDummyDriverForFrontendInvocation;
+  }
+
+  void setIsDummyDriverForFrontendInvocation(bool Value) {
+    IsDummyDriverForFrontendInvocation = Value;
+  }
+
   /// Creates an appropriate ToolChain for a given driver, given the target
   /// specified in \p Args (or the default target). Sets the value of \c
   /// DefaultTargetTriple from \p Args as a side effect.

lib/Driver/DarwinToolChains.cpp

@@ -901,9 +901,15 @@ void
 toolchains::Darwin::validateArguments(DiagnosticEngine &diags,
                                       const llvm::opt::ArgList &args,
                                       StringRef defaultTarget) const {
-  // Validating arclite library path when link-objc-runtime.
-  validateLinkObjcRuntimeARCLiteLib(*this, diags, args);
-  
+  if (!getDriver().isDummyDriverForFrontendInvocation()) {
+    // Validating arclite library path when link-objc-runtime.
+    // If the driver is just set up to retrieve the swift-frontend invocation,
+    // we don't care about link-time, so we can skip this step, which may be
+    // expensive since it might call to `xcrun` to find `clang` and `arclite`
+    // relative to `clang`.
+    validateLinkObjcRuntimeARCLiteLib(*this, diags, args);
+  }
+
   // Validating apple platforms deployment targets.
   validateDeploymentTarget(*this, diags, args);
   validateTargetVariant(*this, diags, args, defaultTarget);

lib/Driver/FrontendUtil.cpp

@@ -75,6 +75,8 @@ bool swift::driver::getSingleFrontendInvocationFromDriverArguments(
   // CompilerInvocation may wish to remap inputs to source buffers.
   TheDriver.setCheckInputFilesExist(false);
 
+  TheDriver.setIsDummyDriverForFrontendInvocation(true);
+
   std::unique_ptr<llvm::opt::InputArgList> ArgList =
     TheDriver.parseArgStrings(ArrayRef<const char *>(Args).slice(1));
   if (Diags.hadAnyError())