[flang] Relax constraints on PURE/ELEMENTAL dummy arguments (#93748)

The standard requires that dummy arguments to PURE functions be
INTENT(IN) or VALUE, but PURE subroutines are allowed to have modifiable
dummy arguments. This makes it impossible to declare atomic operations
as PURE functions, which consequently makes such atomic operations
ineligible for use in parallel constructs and DO CONCURRENT.

This patch downgrades this error to a warning by default, which can be
seen with -pedantic & al. and remain an error with -Werror.