swift234

[build-script] Add flag to export compile commands

CMake supports generation of compilation databases. These have many uses, not
least of which is in supporting code-completion engines like
https://github.com/Valloric/YouCompleteMe.

Add the `--export-compile-commands` flag to `utils/build-script`.
Setting this flag passes `-DCMAKE_EXPORT_COMPILE_COMMANDS=ON` to CMake,
which results in a `compile_commands.json` file being generated along
with built products like `cmark`, `llvm`, and `swift`.