The LLVM sub-module is now fetched even when the LLVM codegen is disabled

cc @Kobzol

Summary

When building rustc with cg_gcc, the LLVM sub-module is fetched while it should not.

Command used

./x test tests/ui/eii --force-rerun

Expected behaviour

The LLVM sub-module should not be fetched and LLVM should not be built when only the GCC codegen is enabled.

Actual behaviour

The LLVM sub-module is fetched and LLVM is built.

Bootstrap configuration (bootstrap.toml)

profile = "compiler"  # Includes one of the default files in src/bootstrap/defaults
change-id = 140732

[build]
jobs = 12

[rust]
codegen-backends = ["gcc"]
deny-warnings = false
debug-assertions = false
debug-assertions-std = false
lto = "off"

[llvm]
download-ci-llvm = false
ninja = false

[gcc]
download-ci-gcc = true

Operating system

ArchLinux

HEAD

b924f94129bce80f36dd9f08bb78dd5393fe196e

Additional context

This was probably introduced by https://github.com/rust-lang/rust/pull/161290.