[Path] Fix off-by-one in finding filename for win style paths (llvm#7…

…8055)

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`.

llvm/lib/Support/Path.cpp

@@ -104,7 +104,7 @@ namespace {
 
     if (is_style_windows(style)) {
       if (pos == StringRef::npos)
-        pos = str.find_last_of(':', str.size() - 2);
+        pos = str.find_last_of(':', str.size() - 1);
     }
 
     if (pos == StringRef::npos || (pos == 1 && is_separator(str[0], style)))

llvm/unittests/Support/Path.cpp

@@ -190,6 +190,7 @@ TEST(Support, Path) {
   paths.push_back("c:\\foo\\");
   paths.push_back("c:\\foo/");
   paths.push_back("c:/foo\\bar");
+  paths.push_back(":");
 
   for (SmallVector<StringRef, 40>::const_iterator i = paths.begin(),
                                                   e = paths.end();