Remove "Skipping LLVM build" section

`download-ci-llvm` is available and enabled by default for all tier 1
platforms, so there is no need to change it. Using system LLVM over CI
llvm is still supported, but is not recommended. Building LLVM from
source is still supported.

This also notes that you need to explicitly disable downloading LLVM
when updating the submodule.

src/backend/updating-llvm.md

@@ -102,6 +102,15 @@ through each in detail.
    LLVM bindings. Note that you should use `#ifdef` and such to ensure that the
    bindings still compile on older LLVM versions.
 
+   Note that `profile = "compiler"` and other defaults set by `x.py setup`
+   download LLVM from CI instead of building it from source. You should
+   disable this temporarily to make sure your changes are being used, by setting
+   ```toml
+   [llvm]
+   download-ci-llvm = false
+   ```
+   in config.toml
+
 4. Test for regressions across other platforms. LLVM often has at least one bug
    for non-tier-1 architectures, so it's good to do some more testing before
    sending this to bors! If you're low on resources you can send the PR as-is

src/building/suggested.md

@@ -201,37 +201,3 @@ git worktree add -b my-feature ../rust2 master
 
 You can then use that rust2 folder as a separate workspace for modifying
 and building `rustc`!
-
-## Skipping LLVM Build
-
-By default, LLVM is built from source, and that takes significant amount of
-time. One way to avoid that is to add this to `config.toml`:
-
-```toml
-[llvm]
-download-ci-llvm = true
-```
-
-Downloading LLVM from CI is still experimental though, and might not be
-available on all platforms. Otherwise, we'd make it a default!
-
-Another alternative is to use LLVM already installed on your computer. This is
-specified in the `target` section of `config.toml`:
-
-```toml
-[target.x86_64-unknown-linux-gnu]
-llvm-config = "/path/to/llvm/llvm-7.0.1/bin/llvm-config"
-```
-
-We have observed the following paths before, which may be different from your system:
-
-- `/usr/bin/llvm-config-8`
-- `/usr/lib/llvm-8/bin/llvm-config`
-
-Note that you need to have the LLVM `FileCheck` tool installed, which is used
-for codegen tests. This tool is normally built with LLVM, but if you use your
-own preinstalled LLVM, you will need to provide `FileCheck` in some other way.
-On Debian-based systems, you can install the `llvm-N-tools` package (where `N`
-is the LLVM version number, e.g. `llvm-8-tools`). Alternately, you can specify
-the path to `FileCheck` with the `llvm-filecheck` config item in `config.toml`
-or you can disable codegen test with the `codegen-tests` item in `config.toml`.