[llvm] [gn] Add libcxx_enable_explicit_modules arg (PR #126017)

[Takuto Ikuta via llvm-commits]

atetubou wrote:

Per the discussion in https://crrev.com/c/6248376/6#message-0ddf8e6a0f3ce1eb1654f7025280d8ed75cf2e81, it is not necessary to use Clang modules for libc++ itself now.

https://github.com/llvm/llvm-project/pull/126017