When rust provides LLVM bitcode files to lld and the bitcode contains
function summaries as used for thin lto, lld defaults to using thin lto.
This prevents some optimizations that are only applied for fat lto.
We solve this by not creating function summaries when fat lto is
enabled. The bitcode for the module is just directly written out.
An alternative solution would be to set the `ThinLTO=0` module flag to
signal lld to do fat lto.
The code in clang that sets this flag is here:
560149b5e3/clang/lib/CodeGen/BackendUtil.cpp (L1150)
The code in LLVM that queries the flag and defaults to thin lto if not
set is here:
e258bca950/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp (L4441-L4446)
Adds a new CI job which checks that the compiler builds with
`--enable-debug` and tests that `needs-force-clang-based-tests` pass
(where cross-language LTO is tested).
With the exception of `tests/run-make/translation/test.rs`, which has a
syntax error.
The expected output in `rustdoc-error-lines/rmake.rs`'s required slight
tweaking.
The two `reproducible-build.rs` files need `// ignore-tidy-linelength`
because rustfmt produces lines longer than 100 chars, which tidy doesn't
like, yuk.
