Saving preprocessed source and/or assembly

In the "How to submit an LLVM bug report" page, we should recommend
that people use gcc -save-temps to write out the .s (llvm assembly)
and .i (preprocessed source code) files, rather than the slightly
more-cumbersome -E/-S methods.

The nice thing about -save-temps is that you need do nothing more than
replace "gcc" with "gcc -save-temps" in a command line, and leave
everything else unchanged. The compilation process runs as before,
except that it leaves behind a foo.s and a foo.i file for each compiled
foo.c file.