Index: clang/lib/Lex/PPDirectives.cpp
===================================================================
--- clang/lib/Lex/PPDirectives.cpp
+++ clang/lib/Lex/PPDirectives.cpp
@@ -2104,6 +2104,27 @@
     StringRef RealPathName = File->getFileEntry().tryGetRealPathName();
     SmallVector<StringRef, 16> Components(llvm::sys::path::begin(Name),
                                           llvm::sys::path::end(Name));
+#if defined(_WIN32)
+    // -Wnonportable-include-path is designed to diagnose includes using
+    // case even on systems with a case-insensitive file system.
+    // On Windows, RealPathName always starts with an upper-case drive
+    // letter for absolute paths, but Name might start with either
+    // case depending on if `cd c:\foo` or `cd C:\foo` was used.
+    // ("foo" will always have on-disk case, no matter which case was
+    // used in the cd command). To not emit this warning solely for
+    // the drive letter, whose case is dependent on if `cd` is used
+    // with upper- or lower-case drive letters, always consider the
+    // given drive letter case as correct for the purpose of this warning.
+    SmallString<128> FixedDriveRealPath;
+    if (llvm::sys::path::is_absolute(Name) &&
+        llvm::sys::path::is_absolute(RealPathName) &&
+        toLowercase(Name[0]) == toLowercase(RealPathName[0]) &&
+        isLowercase(Name[0]) != isLowercase(RealPathName[0])) {
+      assert(Components.size() >= 3 && "should have drive, backslash, name");
+      FixedDriveRealPath = (Name.substr(0, 1) + RealPathName.substr(1)).str();
+      RealPathName = FixedDriveRealPath;
+    }
+#endif
 
     if (trySimplifyPath(Components, RealPathName)) {
       SmallString<128> Path;
Index: llvm/cmake/modules/AddLLVM.cmake
===================================================================
--- llvm/cmake/modules/AddLLVM.cmake
+++ llvm/cmake/modules/AddLLVM.cmake
@@ -1499,7 +1499,6 @@
   "def path(p):\n"
   "    if not p: return ''\n"
   "    p = os.path.join(os.path.dirname(os.path.abspath(__file__)), p)\n"
-  "    if os.name == 'nt' and os.path.isabs(p): return p[0].upper() + p[1:]\n"
   "    return p\n"
   )