Rollup merge of #87049 - jyn514:no-submodules, r=Mark-Simulacrum
Rollup merge of #87049 - jyn514:no-submodules, r=Mark-Simulacrum

Account for `submodules = false` in config.toml when updating LLVM submodule

Fixes https://github.com/rust-lang/rust/issues/86954.

r? ``@Mark-Simulacrum`` cc ``@durin42``

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust