Introduce a minimum CGU size in non-incremental builds.

Because tiny CGUs make compilation less efficient *and* result in worse
generated code.

We don't do this when the number of CGUs is explicitly given, because
there are times when the requested number is very important, as
described in some comments within the commit. So the commit also
introduces a `CodegenUnits` type that distinguishes between default
values and user-specified values.

This change has a roughly neutral effect on walltimes across the
rustc-perf benchmarks; there are some speedups and some slowdowns. But
it has significant wins for most other metrics on numerous benchmarks,
including instruction counts, cycles, binary size, and max-rss. It also
reduces parallelism, which is good for reducing jobserver competition
when multiple rustc processes are running at the same time. It's smaller
benchmarks that benefit the most; larger benchmarks already have CGUs
that are all larger than the minimum size.

Here are some example before/after CGU sizes for opt builds.

- html5ever
  - CGUs: 16, mean size: 1196.1, sizes: [3908, 2992, 1706, 1652, 1572,
    1136, 1045, 948, 946, 938, 579, 471, 443, 327, 286, 189]
  - CGUs: 4, mean size: 4396.0, sizes: [6706, 3908, 3490, 3480]

- libc
  - CGUs: 12, mean size: 35.3, sizes: [163, 93, 58, 53, 37, 8, 2 (x6)]
  - CGUs: 1, mean size: 424.0, sizes: [424]

- tt-muncher
  - CGUs: 5, mean size: 1819.4, sizes: [8508, 350, 198, 34, 7]
  - CGUs: 1, mean size: 9075.0, sizes: [9075]

Note that CGUs of size 100,000+ aren't unusual in larger programs.

compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs

@@ -1385,7 +1385,7 @@ fn vcall_visibility_metadata<'ll, 'tcx>(
     let trait_def_id = trait_ref_self.def_id();
     let trait_vis = cx.tcx.visibility(trait_def_id);
 
-    let cgus = cx.sess().codegen_units();
+    let cgus = cx.sess().codegen_units().as_usize();
     let single_cgu = cgus == 1;
 
     let lto = cx.sess().lto();