Returns a shared translation unit AST for the given translation unit.
	 * <p>
	 * Clients are not allowed to modify the AST and must hold an index read
	 * lock prior to calling this method and continue to hold the lock as long
	 * as the AST is being used.
Returns a shared translation unit AST for the given translation unit.
	 * <p>
	 * Clients are not allowed to modify the AST and must hold an index read
	 * lock prior to calling this method and continue to hold the lock as long
	 * as the AST is being used. | ASTCache.java