[clang-format] Assert that filenames are not empty

[MyDeveloperDay]

Summary

Adds asserts to catch empty filenames, which otherwise will cause a crash in SourceManager.
The clang-format tool now outputs an error if an empty filename is used.
Fixes bug: 34667

[MyDeveloperDay]

Thanks for the patch, I'm sorry it's taken so long sometimes items get lost...This looks to still be relevant, I think you'll need to rebase but it LGTM

$ clang-format --assume-filename= < ../clang-format-examples/test.cpp
Assertion failed: !HasError && "Cannot get value when an error exists!", file C:\llvm/llvm-project/llvm/include\llvm/Support/ErrorOr.h, line 243
Stack dump:
0.      Program arguments: C:\llvm\build\bin\clang-format.exe --assume-filename=
 #0 0x00007ff64d041b55 (C:\llvm\build\bin\clang-format.exe+0x41b55)
 #1 0x00007ff8efdcda2d (C:\WINDOWS\System32\ucrtbase.dll+0x6da2d)
 #2 0x00007ff8efdce901 (C:\WINDOWS\System32\ucrtbase.dll+0x6e901)
 #3 0x00007ff8efdd0261 (C:\WINDOWS\System32\ucrtbase.dll+0x70261)
 #4 0x00007ff8efdd0591 (C:\WINDOWS\System32\ucrtbase.dll+0x70591)
 #5 0x00007ff64d065cdd (C:\llvm\build\bin\clang-format.exe+0x65cdd)
 #6 0x00007ff64d0a11f3 (C:\llvm\build\bin\clang-format.exe+0xa11f3)
 #7 0x00007ff64d081dc3 (C:\llvm\build\bin\clang-format.exe+0x81dc3)
 #8 0x00007ff64d0817bb (C:\llvm\build\bin\clang-format.exe+0x817bb)
 #9 0x00007ff64d00b17c (C:\llvm\build\bin\clang-format.exe+0xb17c)
#10 0x00007ff64d00d515 (C:\llvm\build\bin\clang-format.exe+0xd515)
#11 0x00007ff64d12cc30 (C:\llvm\build\bin\clang-format.exe+0x12cc30)
#12 0x00007ff8efe77974 (C:\WINDOWS\System32\KERNEL32.DLL+0x17974)
#13 0x00007ff8f2caa271 (C:\WINDOWS\SYSTEM32\ntdll.dll+0x6a271)

[MyDeveloperDay]

@jr As there is no response from you, I wonder if you are ok with me taking this over, I'd like to recommend we keep just the:

if (AssumedFileName.empty()) {
    llvm::errs() << "error: empty filenames are not allowed\n";
    return true;
  }

For now, as the other functions where you are adding an assert might be being called from other tooling areas and I don't want to cause others problems, I'd like to keep this change localized, (I hope it's ok but I'll rebase and update the patch)

[MyDeveloperDay]

remove the asserts at least for now.

This fix is important for people using clang-format integrated into some editor tools.

[mitchell-stellar]

LGTM

clang/tools/clang-format/ClangFormat.cpp

if (InvalidBOM) {
     errs() << ".\n";
     return true;
   }
 
   std::vector<tooling::Range> Ranges;
   if (fillRanges(Code.get(), Ranges))
     return true;
   StringRef AssumedFileName = (FileName == "-") ? AssumeFileName : FileName;
+  if (AssumedFileName.empty()) {
+    llvm::errs() << "error: empty filenames are not allowed\n";
+    return true;
+  }
 
   llvm::Expected<FormatStyle> FormatStyle =
       getStyle(Style, AssumedFileName, FallbackStyle, Code->getBuffer());
   if (!FormatStyle) {
     llvm::errs() << llvm::toString(FormatStyle.takeError()) << "\n";
     return true;
   }