[flang][preprocessor] C preprocessor can not be inside of statement

[foxtran]

With the following example,

! example
subroutine test( &
ARG1, &
! test
#ifndef SWAP
#define ARG1 FA
#define ARG2 FB
#else
#define ARG1 FB
#define ARG2 FA
#endif
ARG1, ARG2, &
! test
#undef ARG1
#undef ARG2
&ARG2)
! comment
end

flang-new fails with the following errors:

error: Could not scan test.f90
./test.f90:2:16: error: Unmatched '('
  subroutine test( &
                 ^
./test.f90:12:13: error: bad character ('&') in Fortran token
  ARG1, ARG2, &
              ^
./test.f90:16:1: error: bad character ('&') in Fortran token
  &ARG2)
  ^
./test.f90:16:6: error: Unmatched ')'
  &ARG2)
       ^

Used version (after applying #95332, #93382):

$ flang-new --version
flang-new version 19.0.0git (https://github.com/llvm/llvm-project.git 41587739a63f7622c36715421d215f07d79f9a7d)

[llvmbot]

@llvm/issue-subscribers-flang-frontend

Author: None (foxtran)

With the following example, ``` ! example subroutine test( & ARG1, & ! test #ifndef SWAP #define ARG1 FA #define ARG2 FB #else #define ARG1 FB #define ARG2 FA #endif ARG1, ARG2, & ! test #undef ARG1 #undef ARG2 &ARG2) ! comment end ``` flang-new fails with the following errors: ``` error: Could not scan test.f90 ./test.f90:2:16: error: Unmatched '(' subroutine test( & ^ ./test.f90:12:13: error: bad character ('&') in Fortran token ARG1, ARG2, & ^ ./test.f90:16:1: error: bad character ('&') in Fortran token &ARG2) ^ ./test.f90:16:6: error: Unmatched ')' &ARG2) ^ ```

Used version (after applying #95332, #93382):

$ flang-new --version
flang-new version 19.0.0git (https://github.com/llvm/llvm-project.git 41587739a63f7622c36715421d215f07d79f9a7d)

[klausler]

Is this output from -E close enough to what you want?

#line "./t.F90" 2
      subroutine test(ARG1,FA, FB,ARG2)
#line 18
      end

[klausler]

#96244