ClangImporter: expand -sdk appropriately on Windows

The MSVC environment does not have a `-isysroot` or `-sysroot`
equivalent.  Instead, manually expand the sysroot to the path as a
prefix to `/usr/include` and treat the path as a system include
directory.  This fixes a number of tests which use `-sdk` on Windows.

Author: compnerd

lib/ClangImporter/ClangImporter.cpp

@@ -563,14 +563,21 @@ getNormalInvocationArguments(std::vector<std::string> &invocationArgStrs,
     invocationArgStrs.push_back("-Xclang");
     invocationArgStrs.push_back("-nostdsysteminc");
   } else {
-    // On Darwin, Clang uses -isysroot to specify the include
-    // system root. On other targets, it seems to use --sysroot.
-    if (triple.isOSDarwin()) {
-      invocationArgStrs.push_back("-isysroot");
+    if (triple.isWindowsMSVCEnvironment()) {
+      llvm::SmallString<261> path; // MAX_PATH + 1
+      path = searchPathOpts.SDKPath;
+      llvm::sys::path::append(path, "usr", "include");
+      llvm::sys::path::native(path);
+
+      invocationArgStrs.push_back("-isystem");
+      invocationArgStrs.push_back(path.str());
     } else {
-      invocationArgStrs.push_back("--sysroot");
+      // On Darwin, Clang uses -isysroot to specify the include
+      // system root. On other targets, it seems to use --sysroot.
+      invocationArgStrs.push_back(triple.isOSDarwin() ? "-isysroot"
+                                                      : "--sysroot");
+      invocationArgStrs.push_back(searchPathOpts.SDKPath);
     }
-    invocationArgStrs.push_back(searchPathOpts.SDKPath);
   }
 
   const std::string &moduleCachePath = importerOpts.ModuleCachePath;