pass platform_deps_aliases along

Summary:
`platform_*` attrs in cxx rules are matched against both the current toolchain's
`CxxPlatformInfo` provider `name` and anything in `deps_aliases`.

This diff pipes `platform_deps_aliases` from the `cxx_toolchain` macro down to
the function that creates the underlying provider, just like how
`platform_name` is passed. This enables a cxx toolchain (such as D67764044) to
add these aliases.

Best I could find in the way of documentation for this
> platform_preprocessor_flags: Platform specific preprocessor flags. These
> should be specified as a list of pairs where the first element is an
> un-anchored regex (in java.util.regex.Pattern syntax) against which the
> platform name is matched, and the second element is a list of flags to use
> when preprocessing the target's sources. See preprocessor_flags for more
> information.

Reviewed By: athmasagar

Differential Revision: D68285192

fbshipit-source-id: 1e3ea5fba3aee68addd0cc9c66a2f14b7aa12f34

prelude/cxx/cxx_toolchain.bzl

@@ -172,6 +172,7 @@ def cxx_toolchain_impl(ctx):
     ] + cxx_toolchain_infos(
         internal_tools = ctx.attrs._internal_tools[CxxInternalTools],
         platform_name = platform_name,
+        platform_deps_aliases = ctx.attrs.platform_deps_aliases,
         linker_info = linker_info,
         binary_utilities_info = utilities_info,
         bolt_enabled = value_or(ctx.attrs.bolt_enabled, False),
@@ -247,7 +248,9 @@ def cxx_toolchain_extra_attributes(is_toolchain_rule):
         # Useful when fat and thin toolchahins share the same underlying tools via `command_alias()`,
         # which requires setting up separate platform-specific aliases with the correct constraints.
         "placeholder_tool": attrs.option(dep_type(providers = [RunInfo]), default = None),
-        # Used for resolving any 'platform_*' attributes.
+        # Used for regex matching any 'platform_*' attributes.
+        "platform_deps_aliases": attrs.option(attrs.list(attrs.string()), default = None),
+        # Used for regex matching any 'platform_*' attributes.
         "platform_name": attrs.option(attrs.string(), default = None),
         "private_headers_symlinks_enabled": attrs.bool(default = True),
         "public_headers_symlinks_enabled": attrs.bool(default = True),