Extended Description

In Windows, files are usually case insensitive. The program may not be able to change this, see
http://www.nicklowe.org/2012/02/understanding-case-sensitivity-in-windows-obcaseinsensitive-file_case_sensitive_search/

Meanwhile, SeenFileEntries keys in FileManager are case-sensitive, so we may end up with duplicate mappings in SeenFileEntries for the same file, for example

Hello.h
HELLO.H
hello.H

will all map to the same FileEntry.

Furthermore, the file name in FE will be the searched-for filename rather than the real filename. For example, let's say you have hello.h on disk but you searched for HELLO.H. Since stat succeeds with HELLO.H this will be the filename linked from the FE and used in all clang's diagnostics.

I'm not sure if this should be called a bug but it may not be what the user expects.

Fixing this would require extra work for every file open on Windows, using FindFirstFile looking for file instead of stat checking the file exists.

Reid Kleckner comment on cfe-dev:
I think the correct design is to have multiple strings point to the same FileEntry, which owns the canonical name, rather than pointing to the StringMap key used for the first path encountered.