[DebugInfo] [PDB] Force injected source paths to use backslashes

[mstorsjo]

Summary

This fixes lld/COFF/pdb-natvis.test (which only is run on Windows)
when using paths with forward slashes on Windows.

[rnk]

lgtm

llvm/lib/DebugInfo/PDB/Native/PDBFileBuilder.cpp

 }
 
 void PDBFileBuilder::addInjectedSource(StringRef Name,
                                        std::unique_ptr<MemoryBuffer> Buffer) {
   // Stream names must be exact matches, since they get looked up in a hash
   // table and the hash value is dependent on the exact contents of the string.
   // link.exe lowercases a path and converts / to \, so we must do the same.
   SmallString<64> VName;
-  sys::path::native(Name.lower(), VName);
+  sys::path::native(Name.lower(), VName, sys::path::Style::windows_backslash);
 
   uint32_t NI = getStringTableBuilder().insert(Name);
   uint32_t VNI = getStringTableBuilder().insert(VName);
 
   InjectedSourceDescriptor Desc;
   Desc.Content = std::move(Buffer);
   Desc.NameIndex = NI;
   Desc.VNameIndex = VNI;