RFE: Support saving preprocessed GLSL to a file using the existing `-o <file>` option

It would be useful to be able to save preprocessed GLSL (`-E`) to a file using the `-o ` option.

I'd like to avoid invoking a shell simply to redirect `glslangValidator -E` output to a file.

Start at the glslangValidator command-line handling for the existing -E and -o options. Determine how preprocessed GLSL output is currently emitted, then verify that combining -E with -o writes the output to the requested file without requiring shell redirection.

Written by the indexing model from the issue text.