[ClangImporter] Update umbrella header diagnostic handling

In swiftlang/llvm-project#1860, the diagnostic for a missing import in an umbrella header was improved by moving it to the end of the file and including a fix-it suggesting the import that would be needed. This breaks two things on the Swift side:

• One Swift test assumes the old source location will be used.
• The `ClangSourceBufferImporter` doesn’t work correctly when a diagnostic is emitted at EOF. It tries to create a virtual file covering EOF..<EOF, but it’s not actually valid to start a virtual file at EOF—it would always be empty.

This commit corrects these issues, which should unblock the automerger.

Fixes rdar://69707827.

Author: beccadax

lib/ClangImporter/ClangSourceBufferImporter.cpp

@@ -67,11 +67,17 @@ SourceLoc ClangSourceBufferImporter::resolveSourceLocation(
 
   StringRef presumedFile = presumedLoc.getFilename();
   SourceLoc startOfLine = loc.getAdvancedLoc(-presumedLoc.getColumn() + 1);
-  bool isNewVirtualFile = swiftSourceManager.openVirtualFile(
-      startOfLine, presumedFile, presumedLoc.getLine() - bufferLineNumber);
-  if (isNewVirtualFile) {
-    SourceLoc endOfLine = findEndOfLine(swiftSourceManager, loc, mirrorID);
-    swiftSourceManager.closeVirtualFile(endOfLine);
+
+  // FIXME: Virtual files can't actually model the EOF position correctly, so
+  // if this virtual file would start at EOF, just hope the physical location
+  // will do.
+  if (startOfLine != swiftSourceManager.getRangeForBuffer(mirrorID).getEnd()) {
+    bool isNewVirtualFile = swiftSourceManager.openVirtualFile(
+        startOfLine, presumedFile, presumedLoc.getLine() - bufferLineNumber);
+    if (isNewVirtualFile) {
+      SourceLoc endOfLine = findEndOfLine(swiftSourceManager, loc, mirrorID);
+      swiftSourceManager.closeVirtualFile(endOfLine);
+    }
   }
 
   using SourceManagerRef = llvm::IntrusiveRefCntPtr<const clang::SourceManager>;

test/ClangImporter/diags_from_module.swift

@@ -37,7 +37,7 @@ import Module
 // CHECK-PRIMARY: diags_from_module.swift:[[@LINE-4]]:8: error: could not build Objective-C module 'Module'
 
 // CHECK-WARN: Sub2.h:7:2: warning: here is some warning about something
-// CHECK-WARN: <module-includes>:1:1: warning: umbrella header for module 'Module' does not include header 'NotInModule.h'
+// CHECK-WARN: Module.h:20:1: warning: umbrella header for module 'Module' does not include header 'NotInModule.h'
 // FIXME: show [-Wincomplete-umbrella]
 
 // CHECK-NO-WARN-NOT: warning about something