[CUDA] Initialize our header search using the host triple.

[jlebar]

Summary

This used to work because system headers are found in a (somewhat)
predictable set of locations on Linux. But this is not the case on
MacOS; without this change, we don't look in the right places for our
headers when doing device-side compilation on Mac.

clang/lib/Frontend/CompilerInstance.cpp

void CompilerInstance::createPreprocessor(TranslationUnitKind TUKind) {
   // Apply remappings to the source manager.
   InitializeFileRemapping(PP->getDiagnostics(), PP->getSourceManager(),
                           PP->getFileManager(), PPOpts);
 
   // Predefine macros and configure the preprocessor.
   InitializePreprocessor(*PP, PPOpts, getPCHContainerReader(),
                          getFrontendOpts());
 
-  // Initialize the header search object.
+  // Initialize the header search object.  In CUDA compilations, we use the aux
+  // triple (the host triple) to initialize our header search, since we need to
+  // find the host headers in order to compile the CUDA code.
+  const llvm::Triple *HeaderSearchTriple = &PP->getTargetInfo().getTriple();
+  if (PP->getTargetInfo().getTriple().getOS() == llvm::Triple::CUDA &&
+      PP->getAuxTargetInfo())
+    HeaderSearchTriple = &PP->getAuxTargetInfo()->getTriple();
+
   ApplyHeaderSearchOptions(PP->getHeaderSearchInfo(), getHeaderSearchOpts(),
-                           PP->getLangOpts(), PP->getTargetInfo().getTriple());
+                           PP->getLangOpts(), *HeaderSearchTriple);
 
   PP->setPreprocessedOutput(getPreprocessorOutputOpts().ShowCPP);
 
   if (PP->getLangOpts().Modules && PP->getLangOpts().ImplicitModules)
     PP->getHeaderSearchInfo().setModuleCachePath(getSpecificModuleCachePath());
 
   // Handle generating dependencies, if requested.
   const DependencyOutputOptions &DepOpts = getDependencyOutputOpts();

clang/test/Preprocessor/cuda-macos-includes.cu

+// RUN: %clang -cc1 -fcuda-is-device -isysroot /var/empty \
+// RUN:   -triple nvptx-nvidia-cuda -aux-triple i386-apple-macosx \
+// RUN:   -S -fcuda-is-device -v -o /dev/null -x cuda %s 2>&1 | FileCheck %s
+
+// RUN: %clang -cc1 -isysroot /var/empty \
+// RUN:   -triple i386-apple-macosx -aux-triple nvptx-nvidia-cuda \
+// RUN:   -S -fcuda-is-device -v -o /dev/null -x cuda %s 2>&1 | FileCheck %s
+
+// Check that when we do CUDA host and device compiles on MacOS, we check for
+// includes in /System/Library/Frameworks and /Library/Frameworks.
+
+// CHECK-DAG: ignoring nonexistent directory "/var/empty/System/Library/Frameworks"
+// CHECK-DAG: ignoring nonexistent directory "/var/empty/Library/Frameworks"