Do not enforce checks for LLVM's ABI-breaking build settings.

The Swift runtime uses some header-only code from LLVM's ADT classes,
but we do not want to link libSupport into the runtime. These checks rely
on the presence of symbols in libSupport to identify how the code was
built and cause link failures for mismatches. Without linking that library,
we get link failures regardless, so instead, this just disables the checks.

utils/build-script-impl

@@ -669,6 +669,7 @@ function set_build_options_for_host() {
                 -DCOMPILER_RT_ENABLE_TVOS:BOOL=FALSE
                 -DSANITIZER_MIN_OSX_VERSION="${cmake_osx_deployment_target}"
                 -DLLVM_ENABLE_MODULES:BOOL="$(is_llvm_module_build_enabled)"
+                -DLLVM_DISABLE_ABI_BREAKING_CHECKS_ENFORCING:BOOL=TRUE
             )
             if [[ $(is_llvm_lto_enabled) == "TRUE" ]]; then
                 if [[ $(cmake_needs_to_specify_standard_computed_defaults) == "TRUE" ]]; then