lld/COFF: Simplify getOutputPath() using sys::path functions.

Also mention "basename" and "dirname" in Path.h since I tried
to find these functions by looking for these strings. It might
help others find them faster if the comments contain these strings.

No behavior change.

Differential Revision: https://reviews.llvm.org/D69458

Author: nico

lld/COFF/Driver.cpp

@@ -104,10 +104,7 @@ static std::pair<StringRef, StringRef> getOldNewOptions(opt::InputArgList &args,
 
 // Drop directory components and replace extension with ".exe" or ".dll".
 static std::string getOutputPath(StringRef path) {
-  auto p = path.find_last_of("\\/");
-  StringRef s = (p == StringRef::npos) ? path : path.substr(p + 1);
-  const char* e = config->dll ? ".dll" : ".exe";
-  return (s.substr(0, s.rfind('.')) + e).str();
+  return (sys::path::stem(path) + (config->dll ? ".dll" : ".exe")).str();
 }
 
 // Returns true if S matches /crtend.?\.o$/.

llvm/include/llvm/Support/Path.h

@@ -121,6 +121,8 @@ reverse_iterator rend(StringRef path);
 
 /// Remove the last component from \a path unless it is the root dir.
 ///
+/// Similar to the POSIX "dirname" utility.
+///
 /// @code
 ///   directory/filename.cpp => directory/
 ///   directory/             => directory
@@ -295,7 +297,7 @@ StringRef parent_path(StringRef path, Style style = Style::native);
 ///
 /// @param path Input path.
 /// @result The filename part of \a path. This is defined as the last component
-///         of \a path.
+///         of \a path. Similar to the POSIX "basename" utility.
 StringRef filename(StringRef path, Style style = Style::native);
 
 /// Get stem.