[bazel] libc++ missing includes for libc

libc++'s from_chars implementation includes llvm-libc headers:
- libcxx/src/include/from_chars_floating_point.h:
- #include "shared/fp_bits.h"
- #include "shared/str_to_float.h"
- #include "shared/str_to_integer.h"

We're building libc++ via bazel and hitting an issue where the libc headers from shared/ aren't found from the libc++ includes.

`@llvm-project//libc:libcxx_shared_headers` seems like it should work, but that target's libc_header_library macro uses `copts=["-I..."]` for include paths, but `copts` don't propagate to dependents. Only the `includes` attribute propagates via `CcInfo`'s `compilation_context`.

cc @rupprecht @michaelrj-google @lntue