[flang][preprocessing] Handle #include after & line continuation (#93382)

Some applications like to use a CPP-style #include directive to pull in
a common list of arguments, dummy arguments, or COMMON block variables
after a free-form & line continuation marker. This works naturally with
compilers that run an actual cpp pass over the input before doing
anything specific to Fortran, but it's a case that I missed with this
integrated preprocessor.