Add vendored lld to build invocations (#4)

In some cases Mojo expects to be able to run lld internally

Author: keith

mojo/private/mojo_binary_test.bzl

@@ -104,8 +104,9 @@ def _mojo_binary_test_implementation(ctx):
         progress_message = "%{label} compiling mojo object",
         env = {
             "MODULAR_CRASH_REPORTING_ENABLED": "false",
+            "MODULAR_MOJO_MAX_COMPILERRT_PATH": "/dev/null",  # Make sure this fails if accessed
             "MODULAR_MOJO_MAX_LINKER_DRIVER": "/dev/null",  # Make sure this fails if accessed
-            "MODULAR_MOJO_MAX_LLD_PATH": "/dev/null",  # Make sure this fails if accessed
+            "MODULAR_MOJO_MAX_LLD_PATH": mojo_toolchain.lld.path,
             "MODULAR_MOJO_MAX_ORCRT_PATH": "/dev/null",  # Make sure this fails if accessed
             "TEST_TMPDIR": ".",
         },

mojo/private/toolchain.BUILD

@@ -43,6 +43,7 @@ mojo_toolchain(
         name
         for name, _ in _INTERNAL_LIBRARIES
     ] + ([":all_mojopkgs"] if "{INCLUDE_MOJOPKGS}" else []),
+    lld = "bin/lld",
     mojo = "bin/mojo",
     visibility = ["//visibility:public"],
 )

mojo/providers.bzl

@@ -13,6 +13,7 @@ MojoToolchainInfo = provider(
     fields = {
         "all_tools": "All the files that must be available in actions in order for the toolchain to work.",
         "copts": "Additional compiler options to pass to the Mojo compiler.",
+        "lld": "The lld compiler executable to link with",
         "mojo": "The mojo compiler executable to build with",
         "implicit_deps": "Implicit dependencies that every target should depend on, providing either CcInfo, or MojoInfo",
     },

mojo/toolchain.bzl

@@ -4,7 +4,7 @@ load("//mojo:providers.bzl", "MojoInfo", "MojoToolchainInfo")
 
 def _mojo_toolchain_impl(ctx):
     tool_files = [ctx.attr.mojo[DefaultInfo].files]
-    for dep in ctx.attr.implicit_deps:
+    for dep in ctx.attr.implicit_deps + [ctx.attr.lld]:
         tool_files.append(dep[DefaultInfo].default_runfiles.files)
         tool_files.append(dep[DefaultInfo].files_to_run)
 
@@ -13,6 +13,7 @@ def _mojo_toolchain_impl(ctx):
             mojo_toolchain_info = MojoToolchainInfo(
                 all_tools = tool_files,
                 copts = ctx.attr.copts,
+                lld = ctx.executable.lld,
                 mojo = ctx.executable.mojo,
                 implicit_deps = ctx.attr.implicit_deps,
             ),
@@ -26,6 +27,13 @@ mojo_toolchain = rule(
             mandatory = False,
             doc = "Additional compiler options to pass to the Mojo compiler.",
         ),
+        "lld": attr.label(
+            allow_files = True,
+            mandatory = True,
+            executable = True,
+            cfg = "exec",
+            doc = "The lld executable to link with.",
+        ),
         "mojo": attr.label(
             allow_files = True,
             mandatory = True,