[flang] Improve error message output (llvm#102324)

When a local character variable with non-constant length has an
initializer, it's an error in a couple of ways (SAVE variable with
unknown size, static initializer that isn't constant due to conversion
to an unknown length). The error that f18 reports is the latter, but the
message contains a formatted representation of the initialization
expression that exposes a non-Fortran %SET_LENGTH() operation. Print the
original expression in the message instead.

flang/lib/Evaluate/check-expression.cpp

@@ -493,7 +493,7 @@ std::optional<Expr<SomeType>> NonPointerInitializationExpr(const Symbol &symbol,
  } else {
  context.messages().Say(
  "Initialization expression for '%s' (%s) cannot be computed as a constant value"_err_en_US,
- symbol.name(), folded.AsFortran());
+ symbol.name(), x.AsFortran());
  }
  } else if (xType) {
  context.messages().Say(

flang/test/Semantics/structconst02.f90

@@ -14,7 +14,7 @@ end function realfunc
  integer(kind=ik) :: ix = int(0,kind=ik)
  real(kind=rk) :: rx = real(0.,kind=rk)
  complex(kind=zk) :: zx = cmplx(0.,0.,kind=zk)
- !ERROR: Initialization expression for 'cx' (%SET_LENGTH(" ",len)) cannot be computed as a constant value
+ !ERROR: Initialization expression for 'cx' (" ") cannot be computed as a constant value
  character(kind=ck,len=len) :: cx = ' '
  logical(kind=lk) :: lx = .false.
  real(kind=rk), pointer :: rp => NULL()