Index: llvm/cmake/modules/HandleLLVMOptions.cmake
===================================================================
--- llvm/cmake/modules/HandleLLVMOptions.cmake
+++ llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -431,6 +431,16 @@
     -D_UNICODE
   )
 
+  # Allow setting clang-cl's /winsysroot flag.
+  set(LLVM_WINSYSROOT "" CACHE STRING
+    "If set, argument to clang-cl's /winsysroot")
+  if (LLVM_WINSYSROOT)
+    if (NOT CLANG_CL)
+      message(ERROR "LLVM_WINSYSROOT requires clang-cl")
+    endif()
+    append("/winsysroot${LLVM_WINSYSROOT}" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+  endif()
+
   if (LLVM_ENABLE_WERROR)
     append("/WX" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
   endif (LLVM_ENABLE_WERROR)
Index: llvm/utils/gn/build/BUILD.gn
===================================================================
--- llvm/utils/gn/build/BUILD.gn
+++ llvm/utils/gn/build/BUILD.gn
@@ -22,6 +22,9 @@
 
   # The version of host gcc. Ignored if is_clang is true.
   gcc_version = 9
+
+  # Path of sysroot to use.
+  sysroot = ""
 }
 
 assert(!llvm_build_instrumented_coverage || is_clang,
@@ -281,6 +284,11 @@
       ]
     }
   }
+  if (sysroot != "") {
+    assert(current_os == "win", "FIXME: Make sysroot work on non-win")
+    assert(is_clang, "sysroot only works with clang-cl as host compiler")
+    cflags += [ "/winsysroot" + rebase_path(sysroot, root_build_dir) ]
+  }
 
   if (use_ubsan) {
     assert(is_clang && current_os == "linux",
Index: llvm/utils/sysroot.py
===================================================================
--- /dev/null
+++ llvm/utils/sysroot.py
@@ -0,0 +1,66 @@
+#!/usr/bin/env python3
+
+"""Helps manage sysroots."""
+
+import argparse
+import os
+import subprocess
+import sys
+
+
+def make_fake_sysroot(out_dir):
+    def cmdout(cmd):
+        return subprocess.check_output(cmd).decode(sys.stdout.encoding).strip()
+
+    if sys.platform == 'win32':
+        p = os.getenv('ProgramFiles(x86)', 'C:\\Program Files (x86)')
+
+        winsdk = os.getenv('WindowsSdkDir')
+        if not winsdk:
+            winsdk = os.path.join(p, 'Windows Kits', '10')
+            print('%WindowsSdkDir% not set. You might want to run this from')
+            print('a Visual Studio cmd prompt. Defaulting to', winsdk)
+
+        vswhere = os.path.join(
+                p, 'Microsoft Visual Studio', 'Installer', 'vswhere')
+        vcid = 'Microsoft.VisualStudio.Component.VC.Tools.x86.x64'
+        vsinstalldir = cmdout(
+                [vswhere, '-latest', '-products', '*', '-requires', vcid,
+                    '-property', 'installationPath'])
+
+        def mkjunction(dst, src):
+            subprocess.check_call(['mklink', '/j', dst, src], shell=True)
+        os.mkdir(out_dir)
+        mkjunction(os.path.join(out_dir, 'VC'),
+                   os.path.join(vsinstalldir, 'VC'))
+        os.mkdir(os.path.join(out_dir, 'Windows Kits')) 
+        mkjunction(os.path.join(out_dir, 'Windows Kits', '10'), winsdk)
+    else:
+        assert False, "FIXME: Implement on non-win"
+
+    print('Done.')
+    if sys.platform == 'win32':
+        # CMake doesn't like backslashes in commandline args.
+        abs_out_dir = os.path.abspath(out_dir).replace(os.path.sep, '/')
+        print('Pass -DLLVM_WINSYSROOT=' + abs_out_dir + ' to cmake.')
+    else:
+        print('Pass -DCMAKE_SYSROOT=' + abs_out_dir + ' to cmake.')
+
+
+def main():
+    parser = argparse.ArgumentParser(description=__doc__)
+
+    subparsers = parser.add_subparsers(dest='command', required=True)
+
+    makefake = subparsers.add_parser('make-fake',
+            help='Create a sysroot that symlinks to local directories.')
+    makefake.add_argument('--out-dir', required=True)
+
+    args = parser.parse_args()
+
+    assert args.command == 'make-fake'
+    make_fake_sysroot(args.out_dir)
+
+
+if __name__ == '__main__':
+    main()