Return the name of the file that defined SYMBOL.
The value is normally an absolute file name. It can also be nil,
if the definition is not associated with any file. If SYMBOL
specifies an autoloaded function, the value can be a relative
file name without extension.
If TYPE is nil, then any kind of definition is acceptable. If
TYPE is `defun', `defvar', or `defface', that specifies function
definition, variable definition, or face definition only.