Returns a shared AST for the given translation unit and locks it for
	 * exclusive access. An AST obtained from this method has to be released
	 * by calling {@link #releaseSharedAST(IASTTranslationUnit)}.
	 * Subsequent call to this method will block until the AST is released.
Returns a shared AST for the given translation unit and locks it for
	 * exclusive access. An AST obtained from this method has to be released
	 * by calling {@link #releaseSharedAST(IASTTranslationUnit)}.
	 * Subsequent call to this method will block until the AST is released. | ASTCache.java