fix(metadata-generator): Respect CLANG_ENABLE_MODULES

[mbektchiev]

Respect and pass -fmodules flag to metadata generator when clang modules support is enabled in the application

PR Checklist

• [x] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• [x] There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• [x] You have signed the CLA.
• [x] All existing tests are passing: https://github.com/NativeScript/ios-runtime/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• [ ] Tests for the changes are included.

What is the current behavior?

Metadata generator never uses -fmodules.

What is the new behavior?

Metadata generator will use -fmodules when modules support is enabled

refs #1233