[llvm-windres] Resolve the --preprocessor executable in $PATH (#75390)

The llvm::sys::ExecuteAndWait function doesn't resolve the file to be
executed from $PATH - i.e. it is similar to execv(), not execvp().

Due to this, specifying a --preprocessor argument to llvm-windres only
worked if it specified an absolute path to the preprocessor executable.
This was observed as one of the issues in
https://github.com/msys2/MINGW-packages/pull/19157.

Before d2fa6b69, this usage of
--preprocessor seemed to work, because the first argument of Args[] was
ignored and llvm-windres just executed the autodetected clang executable
regardless.

Also improve the error messages printed if preprocessing failed. (If the
preprocessor executable was started but itself returned an error, we
don't get any error string.)