[Support] Resolve symlinks in getMainExecutable() on Windows (#76304)

This makes the Windows implementation for `getMainExecutable()` behave
the same as its Linux counterpart, in regards to symlinks. Previously,
when using `cmake ... -DLLVM_USE_SYMLINKS=ON`, calling this function
wouldn't resolve to the "real", non-symlinked path.

llvm/lib/Support/Windows/Path.inc

@@ -154,7 +154,10 @@ std::string getMainExecutable(const char *argv0, void *MainExecAddr) {
     return "";
 
   llvm::sys::path::make_preferred(PathNameUTF8);
-  return std::string(PathNameUTF8.data());
+
+  SmallString<256> RealPath;
+  sys::fs::real_path(PathNameUTF8, RealPath);
+  return std::string(RealPath);
 }
 
 UniqueID file_status::getUniqueID() const {