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