[flang] Unnecessary diagnostic on zero-length character object

[DanielCChen]

Consider the following code:

character(0), allocatable :: ch
allocate(character(-1) :: ch)
end

Flang diagnoses this case with

error: Semantic errors in t.f
./t.f:2:27: error: Character length of allocatable object in ALLOCATE must be the same as the type-spec
  allocate(character(-1) :: ch)
                            ^^

Because both of the type-spec at the declaration and the allocate statement specify zero-length, the diagnostic message seems unnecessary.
All, gfortran, ifort and XLF don't diagnose this case.

[llvmbot]

@llvm/issue-subscribers-flang-frontend

Author: Daniel Chen (DanielCChen)

Consider the following code: ``` character(0), allocatable :: ch allocate(character(-1) :: ch) end ```

Flang diagnoses this case with

error: Semantic errors in t.f
./t.f:2:27: error: Character length of allocatable object in ALLOCATE must be the same as the type-spec
  allocate(character(-1) :: ch)
                            ^^

Because both of the type-spec at the declaration and the allocate statement specify zero-length, the diagnostic message seems unnecessary.
All, gfortran, ifort and XLF don't diagnose this case.