[Support] [Path] Use std::replace instead of an explicit comparison loop. NFC.

[mstorsjo]

Summary

After 8fc7a907b93a8e9eef96e872f8f926db3ebfe9b6, this loop does
the same as a plain std::replace.

Also clarify the comment about what this function does.

[thakis]

lg

llvm/include/llvm/Support/Path.h

void append(SmallVectorImpl<char> &path, const_iterator begin,
             const_iterator end, Style style = Style::native);
 
 /// @}
 /// @name Transforms (or some other better name)
 /// @{
 
 /// Convert path to the native form. This is used to give paths to users and
 /// operating system calls in the platform's normal way. For example, on Windows
-/// all '/' are converted to '\'.
+/// all '/' are converted to '\'. On Unix, it converts all '\' to '/'.
 ///
 /// @param path A path that is transformed to native format.
 /// @param result Holds the result of the transformation.
 void native(const Twine &path, SmallVectorImpl<char> &result,
             Style style = Style::native);
 
 /// Convert path to the native form in place. This is used to give paths to
 /// users and operating system calls in the platform's normal way. For example,

llvm/lib/Support/Path.cpp

if (real_style(style) == Style::windows) {
     std::replace(Path.begin(), Path.end(), '/', '\\');
     if (Path[0] == '~' && (Path.size() == 1 || is_separator(Path[1], style))) {
       SmallString<128> PathHome;
       home_directory(PathHome);
       PathHome.append(Path.begin() + 1, Path.end());
       Path = PathHome;
     }
   } else {
-    for (auto PI = Path.begin(), PE = Path.end(); PI < PE; ++PI)
-      if (*PI == '\\')
-        *PI = '/';
+    std::replace(Path.begin(), Path.end(), '\\', '/');
   }
 }
 
 std::string convert_to_slash(StringRef path, Style style) {
   if (real_style(style) != Style::windows)
     return std::string(path);
 
   std::string s = path.str();