[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174385 91177308-0d34-0410-b5e6-96231b3b80d8