Fix/deps betterc semantic3

[zxan]

Fixes #20392

Problem

Using -deps with -betterC causes spurious errors in transitively imported druntime modules the user never references. A bare unused import is enough to trigger it:

import core.memory;

dmd repro.d -betterC -deps=deps.txt // fails with throw/GC errors in core.exception, core.time
dmd repro.d -betterC -c // compiles clean

Root cause

When -deps is active, tryMain calls semantic3OnDependencies(), which recursively forces full function-body analysis (semantic3) on every transitively imported module. Under -betterC, that flags constructs in unreferenced druntime functions that are legal in druntime but illegal in user code. The -deps output is populated during normal semantic passes and does not require this extra step.

Confirmed via GDB stack trace:
#40 dmd.dsymbolsem.semantic3OnDependencies(Module m) at src/dmd/dsymbolsem.d:718
#41 dmd.dsymbolsem.semantic3OnDependencies(Module m) at src/dmd/dsymbolsem.d:721
#42 dmd.main.tryMain(...) at src/dmd/main.d:732

Fix

Remove semantic3OnDependencies and its call site in main.d. This revives the approach from #9122, which stalled in 2019 due to a since-resolved objection. For recursive dependencies, use -deps combined with -i.

@dkorpel

[dkorpel]

A test case should be minimal and self-contained. Importing druntime creates a very large deps file which is fragile, as any change could alter that output and require updating the this test.

[dkorpel]

This looks like an AI-written description that is very literal. It describes the issue from a very technical lens through the specific issue that this PR closes, which is only a manifestation of the more general problem that -deps conflates -i.

[zxan]

Removed the druntime repro and the stray deps.txt. Added compilable/test20392.d, which imports a local module with a static assert(0) inside a function body. It fails on the old compiler with -deps and passes now, no druntime involved. Also rewrote the changelog around the -deps/-i conflation instead of the betterC symptom.