Switch to the new C++ toolchains API

[AustinSchuh]

The new BUILD file based C++ toolchains promise to be much easier to maintain.

https://github.com/bazelbuild/rules_cc/tree/main/examples/rule_based_toolchain

What do you all think about switching over to using those? I'm happy to help.

The example makes it look like we can reuse pieces from rules_cc.

cc_toolchain(                                                                                                                           
    name = "host_clang",                                                                                                           
    args = select({                                                                                                                              
        "@platforms//os:linux": [                                                                                                                            
            "//toolchain/args:linux_sysroot",                                                                             
        ],                                                                                                                                         
        "//conditions:default": [],    
    }) + [                                                                                                                                                       
        "//toolchain/args:no_canonical_prefixes",                                                                                                   
        "//toolchain/args:warnings",                                                                                  
    ],                                                                    
    enabled_features = ["@rules_cc//cc/toolchains/args:experimental_replace_legacy_action_config_features"],                                                                                                  
    known_features = ["@rules_cc//cc/toolchains/args:experimental_replace_legacy_action_config_features"],    
    tool_map = "//toolchain/tools:all_tools",                                                                       
)                                                                                                                                                            
                                                                                                                 
toolchain(                                                                                                                         
    name = "host_cc_toolchain",                                                                                                   
    toolchain = ":host_clang",                                                                                                                                    
    toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",                                                                                
)  

[keith]

note that this repo mostly just doesn't use any of the old stuff for this either since it just shells out to the default toolchain setup code.

toolchains_llvm/toolchain/cc_toolchain_config.bzl

Line 368 in bb3f7d0

unix_cc_toolchain_config(

[AustinSchuh]

Yup. I'm trying to upstream a bunch of fixes we've built up over the years that let us cross compile against sysroots. I'm quickly finding all the places where new flags need to be injected in very specific spots through unix_cc_toolchain_config which don't exist.