[Test Suite] Pull generateSource into lldbtest

Summary:
Convert tests using LLDB headers to use generateSource to put the right include paths in place regardless of whether or not you're building a framework.

This also abstracted generateSource out of TestPublicAPIHeaders.py into lldbtest.py.

Reviewers: tfiala, zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D25887

llvm-svn: 285357