buck2: add a buck2-only linker wrapper rule that preserves exe / flag…

…s difference

Summary:
Doing this to help D48329688. For the Go rules, they want to be able to separate linker cmd and linker flags, but here we
have a bunch of linker flags that actually come via the linker cmd!

This fixes that by allowing the linker wrappers in fbcode to expose the separation between cmd and flags, and then the
toolchain implementation can place the flags where they belong.

Reviewed By: cjhopman

Differential Revision: D48899332

fbshipit-source-id: fdfd117996ae556d70b845b569ca1d6bb1ecb965

prelude/cxx/cxx_link_utility.bzl

@@ -185,7 +185,7 @@ def executable_shared_lib_arguments(
 
     return (extra_args, runtime_files, shared_libs_symlink_tree)
 
-def cxx_link_cmd_parts(ctx: AnalysisContext) -> (RunInfo.type, cmd_args):
+def cxx_link_cmd_parts(ctx: AnalysisContext) -> ((RunInfo.type | cmd_args), cmd_args):
     toolchain = get_cxx_toolchain_info(ctx)
 
     # `toolchain_linker_flags` can either be a list of strings, `cmd_args` or `None`,

prelude/cxx/cxx_toolchain_types.bzl

@@ -7,6 +7,12 @@
 
 load("@prelude//cxx:debug.bzl", "SplitDebugMode")
 
+# For cases where our `ld` dependency provides more than an executable and
+# would like to give us flags too. We use this to place the flags in the proper
+# field (linker_flags), so that things that want ldflags without the linker
+# executable can access those.
+RichLinkerRunInfo = provider(fields = ["exe", "flags"])
+
 LinkerType = ["gnu", "darwin", "windows"]
 
 ShlibInterfacesMode = enum("disabled", "enabled", "defined_only")