main: remove -no-debug flag

The -no-debug flag controls whether to emit DWARF debug information.
However, instead of limiting debug information altogether, I think it's
better to strip it off at the end if it isn't needed. For several
reasons:

  * Some parts of the compiler now rely on the presence of debug
    information for proper diagnostics.
  * It works better with the cache: there is no distinction between
    debug and no-debug builds.
  * It makes it easier (or possible at all) to enable debug information
    in the wasi-libc library without big downsides.

I'm doing this in a separate commit so this can be reverted if needed.

compileopts/options.go

@@ -29,7 +29,6 @@ type Options struct {
 	DumpSSA         bool
 	VerifyIR        bool
 	PrintCommands   func(cmd string, args ...string)
-	EmitDWARF       bool
 	Debug           string
 	PrintSizes      string
 	PrintAllocs     *regexp.Regexp // regexp string