[Lex] Use line markers in preprocessed assembly predefines file

GNU line marker directives are not recognised when preprocessing
assembly files, meaning they can't be used in the predefines file
meaning macros defined on the command line are reported as being
built-in.

Change this to permit line markers but only in the predefines file,
so we can correctly report command line macros as coming from the
command line.

Differential Revision: https://reviews.llvm.org/D145397