[flang] Flang fails to scan a Fortran file with preprocessor macros

[inaki-amatria]

• flang version is: flang version 20.0.0git (https://github.com/llvm/llvm-project.git 6233346895abfb57782511cddc263d439fdd537b)
• Reduced test case:

! foo.F
#define BAR(A, B) ((A) * 10000 + (B) * 100)
#define FOO(A, B, C, AA, BB) ( \
  (defined(AA) && defined(BB)) && \
  (BAR(A, B) C BAR(AA, BB)))

#if FOO(9, 5, >, BAZ, FUX)
#endif

• flang invocation is: flang -c foo.F
• Expected behavior is: The code compiles successfully.
• Actual behavior: Compilation aborts with the following error:

error: Could not scan foo.F
foo.F:6:5: error: expected ')'
  #if FOO(9, 5, >, BAZ, FUX)
      ^^^^^^
foo.F:2:30: in a macro defined here
  #define FOO(A, B, C, AA, BB) ( \
                               ^^
that expanded to:
  ( (defined(BAZ) && defined(FUX)) && (((9) * 10000 + ( 5) * 100) > ((BAZ) * 10000 + ( FUX) * 100)))
            ^

• Additional info: The following, further reduced, test case also reproduces the issue:

#define FOO() (defined(BAR))

#if FOO()
#endif

[llvmbot]

@llvm/issue-subscribers-flang-frontend

Author: Iñaki Amatria Barral (inaki-amatria)

- `flang` version is: `flang version 20.0.0git (https://github.com/llvm/llvm-project.git 6233346)` - Reduced test case: ```f90 ! foo.F #define BAR(A, B) ((A) * 10000 + (B) * 100) #define FOO(A, B, C, AA, BB) ( \ (defined(AA) && defined(BB)) && \ (BAR(A, B) C BAR(AA, BB)))

#if FOO(9, 5, >, BAZ, FUX)
#endif

- `flang` invocation is: `flang -c foo.F`
- Expected behavior is: The code compiles successfully.
- Actual behavior: Compilation aborts with the following error:

error: Could not scan foo.F
foo.F:6:5: error: expected ')'
#if FOO(9, 5, >, BAZ, FUX)
^^^^^^
foo.F:2:30: in a macro defined here
#define FOO(A, B, C, AA, BB) (
^^
that expanded to:
( (defined(BAZ) && defined(FUX)) && (((9) * 10000 + ( 5) * 100) > ((BAZ) * 10000 + ( FUX) * 100)))
^

- Additional info: The following, further reduced, test case also reproduces the issue:
```f90
#define FOO() (defined(BAR))

#if FOO()
#endif