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

After 8fc7a907b9, this loop does
the same as a plain `std::replace`.

Also clarify the comment about what this function does.

Differential Revision: https://reviews.llvm.org/D111730

llvm/include/llvm/Support/Path.h

@@ -212,7 +212,7 @@ void append(SmallVectorImpl<char> &path, const_iterator begin,
 
 /// 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.

llvm/lib/Support/Path.cpp

@@ -557,9 +557,7 @@ void native(SmallVectorImpl<char> &Path, Style style) {
       Path = PathHome;
     }
   } else {
-    for (auto PI = Path.begin(), PE = Path.end(); PI < PE; ++PI)
-      if (*PI == '\\')
-        *PI = '/';
+    std::replace(Path.begin(), Path.end(), '\\', '/');
   }
 }