[Path] Dix off-by-one in finding filename for win style paths

This fixes a crash where `path::parent_path` causes an invalid
access on a string upon receiving a path that consists of a single colon.

On Windows machine, with runtime checks enabled build, upon `clang -I: test.cc` produces:
```
Assertion failed: Index < Length && "Invalid index!", file llvm\include\llvm/ADT/StringRef.h, line 232
...
 #6 0x00007ff7816201eb `anonymous namespace'::parent_path_end llvm\lib\Support\Path.cpp:144:0
 #7 0x00007ff781620135 llvm::sys::path::parent_path(class llvm::StringRef, enum llvm::sys::path::Style) llvm\lib\Support\Path.cpp:470:0
```

Ideally, we can look for the last colon starting from the last
character, but we can instead start from second to last, and handle
empty paths by abusing `0 - 1 == npos`.