rustc_session exports it for other crates to avoid mismatching
crate versions.

The code was broken because it printed "llvm-config" instead of the
absolute path to the llvm-config executable, causing Cargo to always
rebuild librustc_llvm if using system LLVM.

Also, it's not the build system's job to rebuild when a system library
changes, so we simply don't emit "rerun-if-changed" if a path to LLVM
was not explicitly provided.

x.py sets it unconditionally, so want it for plain "cargo build".

We need to load one of the panic runtimes that is in src (vs. pre-built in the
compiler's sysroot) to ensure that we don't load libpanic_unwind from the
sysroot. That would lead to a load of libcore, also from the sysroot, and create
lots of errors about duplicate lang items.

The compiler normalizes source when reading files initially (removes BOMs, etc), but not when loading external sources.

Fixes rust-lang#70874 by normalizing when loading external sources too. Adds a test to verify normalization.

I was looking into improving some Clippy documentation but was missing a
place that explains the `kw` and `sym` modules from rustc.

…-Simulacrum

Partial work on building with Cargo

This cherry picks the commits I'm directly approving from rust-lang#70999, I want to land them so that that PR is smaller.

…an-DPC

Clean up E0515 explanation

r? @Dylan-DPC

…schievink

Update links of `rustc guide`

Picks up the things we left behind in the transition, hopefully they're last ones.

r? @spastorino

Normalize source when loading external foreign source into SourceMap

The compiler normalizes source when reading files initially (removes BOMs, etc), but not when loading external sources.

This leads to the external source matching according to the `src_hash`, but differing internally because it was not normalized.

Fixes rust-lang#70874.

…-DPC

Add some basic docs to `sym` and `kw` modules

I was looking into improving some Clippy documentation but was missing a
place that explains the `kw` and `sym` modules from rustc.

This adds some very basic usage documentation to these modules.

…an-DPC

Clean up E0516 explanation

r? @Dylan-DPC