The compiler-rt dependency was removed in
7e6c9f3 in favor of a vendored
dependency on rust-lang/compiler-builtins (dual UIUC and MIT licensed).
That vendored dependency was converted to a regular Cargo dependency in
4c21a3b.

Use of libbacktrace was removed in
06d565c where we switched to using the
gimili library instead. Note: the backtrace submodule located at
library/backtrace points to backtrace-rs which removed support for using
libbacktrace in rust-lang/backtrace-rs#423.

The rustc_apfloat crate is a port of LLVM's APFloat code and was
introduced in rust-lang#43554. At that
time, LLVM was licensed under the UIUC license.

Since that time, contributors have made changes to the code without
being advised of the UIUC licensing resulting in the code base being a
mixture of UIUC licensed code and Apache 2/MIT licensed code.

Going forward, the compiler team will be asking contributors to license
their contributions under all of Apache 2/MIT/UIUC licenses so that,
eventually, either we can relicense the crate as simply UIUC licensed or
create a new port that is Apache 2/MIT licensed which can use the
changes contributed on top of the UIUC base.

LLVM has relicensed their codebase. Remove the old UIUC license text
from the LLVM section and include the licensing text used by LLVM
itself.