Fortran INCLUDE lines have (until now) been treated like #include
directives. This isn't how things work with other Fortran compilers when
running under the -E option for preprocessing only, so stop doing it by
default, and add -fpreprocess-include-lines to turn it back on when
desired.
Fix some problems with INCLUDE line recognition pointed out by some
recently-added tests to the LLVM test suite.
Differential Revision: https://reviews.llvm.org/D155497