Rollup merge of #110074 - majaha:config_toml_fix, r=jyn514

Make the "codegen" profile of `config.toml` download and build llvm from source.

The stated purpose of the codegen profile in config.toml is:

> `# These defaults are meant for contributors to the compiler who modify codegen or LLVM`

but `download-ci-llvm` must be set to be false for the llvm source to even be downloaded. This patch adds that in.

Also included: a small docs fix in `config.example.toml`

src/bootstrap/defaults/config.codegen.toml

@@ -9,6 +9,8 @@ compiler-docs = true
 assertions = true
 # enable warnings during the llvm compilation
 enable-warnings = true
+# build llvm from source
+download-ci-llvm = false
 
 [rust]
 # This enables `RUSTC_LOG=debug`, avoiding confusing situations