Factor libc++_host use into llvm_host_defaults

The llvm-build-host-tools-defaults defaults module now includes two
more properties:

    shared_libs: ["libc++_host"],
    stl: "none",

This change has no effect on vndk/tools/header-checker:

 * The header-checker-defaults module adds a use of libc++_host. This
   addition counteracts the removal in header-abi-dumper,
   header-abi-linker-defaults, libheader-checker, and
   header-checker-unittests

 * The header-abi-linker-defaults module is unaffected.

Consolidating these settings allows a CL in
prebuilts/clang/host/linux-x86 to remove libc++_host in one place.

Bug: http://b/332594828
Test: prebuilts/clang-tools/build-prebuilts.sh (in aosp/clang-tools repo)
Test: out/soong/dist/bin/header-abi-dumper --help
Change-Id: I159b9f59609a9982b4f3b5f4ee432c281822c622
2 files changed