Add -Z external-clangrt

This adds the unstable `-Z external-clangrt` flag that will prevent
rustc from emitting linker paths for the in-tree LLVM sanitizer runtime
library.

Author: chriswailes

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -1192,6 +1192,7 @@ fn add_sanitizer_libraries(sess: &Session, crate_type: CrateType, linker: &mut d
     // are currently distributed as static libraries which should be linked to
     // executables only.
     let needs_runtime = !sess.target.is_like_android
+        && !sess.opts.unstable_opts.external_clangrt
         && match crate_type {
             CrateType::Executable => true,
             CrateType::Dylib | CrateType::Cdylib | CrateType::ProcMacro => sess.target.is_like_osx,

compiler/rustc_session/src/options.rs

@@ -1621,6 +1621,8 @@ options! {
         "emit the bc module with thin LTO info (default: yes)"),
     export_executable_symbols: bool = (false, parse_bool, [TRACKED],
         "export symbols from executables, as if they were dynamic libraries"),
+    external_clangrt: bool = (false, parse_bool, [UNTRACKED],
+        "rely on user specified linker commands to find clangrt"),
     extra_const_ub_checks: bool = (false, parse_bool, [TRACKED],
         "turns on more checks to detect const UB, which can be slow (default: no)"),
     #[rustc_lint_opt_deny_field_access("use `Session::fewer_names` instead of this field")]