Add a filegroup containing _all_ sources to the libc build rules

These rules already expose a filegroup containing the _dependencies_,
but that misses the source files directly in the top level library.

Without this filegroup, there isn't a way to access the source files
used by libcxx when building it, etc.

Author: chandlerc

utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl

@@ -247,6 +247,12 @@ def libc_header_library(name, hdrs, deps = [], **kwargs):
         **kwargs
     )
 
+    _libc_srcs_filegroup(
+        name = name + "_hdrs",
+        libs = [":" + name],
+        enforce_headers_only = True,
+    )
+
 def libc_generated_header(name, hdr, yaml_template, other_srcs = []):
     """Generates a libc header file from YAML template.