middle-end/105376 - invalid REAL_CST for DFP constant

middle-end/105376 - invalid REAL_CST for DFP constant

We are eventually ICEing in decimal_to_decnumber on non-decimal
REAL_VALUE_TYPE that creep in from uses of build_real (..., dconst*)
for DFP types.  The following extends the decimal_to_decnumber
special-casing of dconst* to build_real, avoiding the bogus REAL_CSTs
from creeping into the IL and modified to ones not handled by
the decimal_to_decnumber special casing.  It also makes sure to
ICE for not handled dconst* values at the point we build the REAL_CST.

2022-04-27  Richard Biener  <rguenther@suse.de>

	PR middle-end/105376
	* tree.cc (build_real): Special case dconst* arguments
	for decimal floating point types.

	* gcc.dg/pr105376.c: New testcase.

(cherry picked from commit e27eef7478f30ea79048dbde3317e89679d75a6e)