config.toml: Allow passing arbitrary configure and build args to LLVM cmake

[tgross35]

LLVM's build system exposes a lot of flags that may be nice to tweak, but we probably don't want to worry about mapping directly in config.toml. It would be helpful to support something like the following:

[llvm]
configure-args = [
    "-DCMAKE_C_COMPILER_LAUNCHER=sccache",
    "-DCMAKE_CXX_COMPILER_LAUNCHER=sccache",
    "-DLLVM_OPTIMIZED_TABLEGEN=true",
    "-DBUILD_SHARED_LIBS=true", # unsure whether this would work with Rust
]
build-args = [
    # ...
]

These would get passed to build_arg and configure_arg on cmake::Config.

[tgross35]

It might also be good to just enable -DLLVM_OPTIMIZED_TABLEGEN=true by default when debug = true is set for the build time boost.

[moxian]

Doesn't build-config introduced in #93756 do exactly this already?

[tgross35]

I guess so, I could never find that option. Thanks for pointing it out.