[flang] Fix printing of constc and parsing of #fir.real

Printing and parsing of constc didn't agree with each other. This patch
treats the parsing of constc as the final word and fixes the printing
accordingly.

More concretely, this patch prints the RealAttrs that make up the
ConstcOp directly instead of casting to mlir::FloatAttr (which blows
up). It also fixes parseFirRealAttr to invoke APFloat's method for
getting the size of a floating point type instead of computing it as
8 * kind (which blows up for BFloat, with kind == 3 and size == 16).

Kudos to Kiran Chandramohan <kiran.chandramohan@arm.com> for noticing
that we were missing tests for constc in fir-ops.fir.

Differential Revision: https://reviews.llvm.org/D114081