Compute indent never relative to current column

Previously the pretty printer would compute indentation always relative
to whatever column a block begins at, like this:

    fn demo(arg1: usize,
            arg2: usize);

This is never the thing to do in the dominant contemporary Rust style.
Rustfmt's default and the style used by the vast majority of Rust
codebases is block indentation:

    fn demo(
        arg1: usize,
        arg2: usize,
    );

where every indentation level is a multiple of 4 spaces and each level
is indented relative to the indentation of the previous line, not the
position that the block starts in.

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -655,7 +655,7 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
         // Outer-box is consistent.
         self.cbox(INDENT_UNIT);
         // Head-box is inconsistent.
-        self.ibox(w.len() + 1);
+        self.ibox(0);
         // Keyword that starts the head.
         if !w.is_empty() {
             self.word_nbsp(w);