Added some documentation and minor cleanups.

utils/bazel/llvm-project-overlay/llvm/BUILD.bazel

 # toolchain or the `.bazelrc` file. This is just a workaround until we have a
 # widely available feature to enable unlimited stack frame instead of using
 # this `Make` variable.
 llvm_copts = [
     "$(STACK_FRAME_UNLIMITED)",
 ]
 
 # A command-line flag for restricting the list of LLVM targets to build.
+#
+# See `llvm_targets` in `:targets.bzl` for the full list of available targets.
+#
+# Usage:
+#
+# $ bazel build --@llvm-projectllvm:targets=AArch64,X86 --config=generic_clang @llvm-project...
 string_list_flag(
     name = "targets",
     build_setting_default = llvm_targets,
 )
 
 # For each potential LLVM target, a configuration setting for whether that
 # target is enabled.
 [
     config_setting(
         name = "%s_enabled" % target,
         flag_values = {":targets": target},
     )
     for target in llvm_targets
 ]
 
-# Replace macros in headers with lists of enabled targets.
+# Expand LLVM target enumerations in C++ headers with lists of enabled targets.
 [
     enum_targets_gen(
         name = name,
         src = "include/llvm/Config/{}.def.in".format(path),
         out = "include/llvm/Config/{}.def".format(path),
         macro_name = macro_name,
         valid_targets = targets_with_file(pattern),
         targets = ":targets",

utils/bazel/llvm-project-overlay/llvm/enum_targets_gen.bzl

enum_targets_gen = rule(
     implementation = _enum_targets_gen_impl,
     attrs = {
         "src": attr.label(
             mandatory = True,
             allow_single_file = True,
         ),
         "valid_targets": attr.string_list(
             mandatory = True,
-            doc = "A list of LLVM targets that are valid for this enum.",
+            doc = "A list of LLVM targets that are valid for this enum. " +
+                  "Since this value depends on what files are present, it " +
+                  "must be evaluated outside the implementation of the rule.",
         ),
         "targets": attr.label(
             mandatory = True,
             doc = "A configuration of LLVM targets to build.",
         ),
         "out": attr.output(mandatory = True),
         "macro_name": attr.string(
             mandatory = True,

utils/bazel/llvm-project-overlay/llvm/target_macros.bzl

def targets_with_file(suffix):
     The sources for LLVM targets are in folders `lib/Target/*`.  This macro
     finds all targets that contain a file matching the suffix, ie.,
     `lib/Target/<target>/<suffix>`.
     """
     matches = native.glob(["lib/Target/*/{}".format(suffix)])
     return [m.split("/")[2] for m in matches]
 
 def enabled_targets(pattern):
-    """Build a list of Bazel targets matching the pattern.
+    """Build a list of enabled LLVM targets.
 
-    This macro builds an expression that evaluates to a list of enabled
-    Bazel targets of the form `<target><pattern>`.
+    This macro builds a depency list that evaluates to Bazel targets of the
+    form `<target><pattern>` for every enabled LLVM target.
     """
     # Build a list of selects that either choose the Bazel target or not,
     # depending on the `:targets` flag.
     selects = [
         select({
             ":%s_enabled" % t: ["{}{}".format(t, pattern)],
             "//conditions:default": [],
         })
         for t in llvm_targets
     ]
     # Fold over the selects with "+".
-    result = selects[0]
-    for s in selects[1:]:
+    result = []
+    for s in selects:
       result += s
     return result