Mixed-source cflags (#41049)

Allow flags from different sources (compilers, `require:`, command-line
specs, and `depends_on`) to be merged together, and enforce a consistent
order among them.

The order is based on the sources, e.g. flags on specs from the command
line always come last. Some flag order consistency issues are fixed:

1. Flags from `compilers.yaml` and the command line were always intra- and
   inter-source order consistent.
2. Flags from dependents and packages.yaml (introduced via `require:`)
   were not: for `-a -b` from one source and `-c` from another, the final
   result might rearrange `-a -b`, and would also be inconsistent in terms
   of whether `-c` came before or after.

(1) is/was handled by going back to the original source, i.e., flags are
retrieved directly from the command line spec rather than the solver.

(2) is addressed by:

* Keeping track of grouped flags in the solver
* Keeping track of flag sources in the solver on a per-flag basis

The latter info is used in this PR to enforce DAG ordering on flags
applied from multiple dependents to the same package, e.g., for this
graph:

```
   a
  /|\
 b | c
  \|/
   d
```

If `a`, `b`, and `c` impose flags on `d`, the combined flags on `d` will
contain the flags of `a`, `b`, and `c` -- in that order. 

Conflicting flags are allowed (e.g. -O2 and -O3). `Spec.satisifes()` has
been updated such that X satisfies Y as long as X has *at least* all of
the flags that Y has. This is also true in the solver constraints.
`.satisfies` does not account for how order can change behavior (so
`-O2 -O3` can satisfy `-O3 -O2`); it is expected that this can be
addressed later (e.g. by prohibiting flag conflicts).

`Spec.constrain` and `.intersects` have been updated to be consistent
with this new definition of `.satisfies`.

0 files changed, 0 insertions, 0 deletions