Build XHPAST earlier in the stack

Summary:
Currently, the PhutilLibraryMapBuilder will fail in an obscure way if the XHPAST binary is not built. The issue is that phutil_symbols.php will attempt to build the XHPAST binary, and this will happen eight times concurrently. Ultimately, this will fail with:

<libphutil>/scripts/build_xhpast.sh: line 24: ./xhpast: Text file busy

Test Plan: See D11528.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11627