[MC] Always lower .fill to MCFillFragment (#194164)
Constant-count, constant-pattern .fill expands inline into the current
fragment via emitIntValue per byte, wasting both memory and time (a
redundant copy at MCAssembler.cpp). #50974 reports a 4s compile dropping
to 0.6s when the loop is removed.
Drop the inline path so .fill always becomes MCFillFragment.
This cannot be done before commit 507efbcce03d (2023) allowed
label differences to be separated by a MCFillFragment.
In directive_fill.s, the parse time warning is now diagnosed by
MCAssembler.
