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.

compiler/symbol.go

@@ -386,7 +386,7 @@ func (c *compilerContext) getGlobal(g *ssa.Global) llvm.Value {
 			llvmGlobal.SetAlignment(alignment)
 		}
 
-		if c.Debug && !info.extern {
+		if !info.extern {
 			// Add debug info.
 			pos := c.program.Fset.Position(g.Pos())
 			diglobal := c.dibuilder.CreateGlobalVariableExpression(c.difiles[pos.Filename], llvm.DIGlobalVariableExpression{