[frontend] Don't put a PCH/PTH filename into the set of includes in the preprocessor options;
since only one of them is allowed in command-line, process them separately.

Otherwise, if more than one is specified in the command-line, one is processed normally
and the others are going to be treated and included as header files.

Related to radar://13140508
[arcmt] Make sure the objc migrators work fine when used with a PCH.

rdar://13140508