support clang 19 only

[zaucy]

llvm 19 now has std modules installed for libc++ by default. I'm going to support clang 19 only for this reason and remove support for the env var to configure libc++ for simplicitly sake.

clang-19 -print-file-name=libc++.modules.json

example output

/usr/lib/llvm-19/bin/../lib/libc++.modules.json

json file contents

{
  "version": 1,
  "revision": 1,
  "modules": [
    {
      "logical-name": "std",
      "source-path": "../share/libc++/v1/std.cppm",
      "is-std-library": true,
      "local-arguments": {
        "system-include-directories": [
          "../share/libc++/v1"
        ]
      }
    },
    {
      "logical-name": "std.compat",
      "source-path": "../share/libc++/v1/std.compat.cppm",
      "is-std-library": true,
      "local-arguments": {
        "system-include-directories": [
          "../share/libc++/v1"
        ]
      }
    }
  ]
}