This is an alternate fix for the issue described in commit be63facd, whose
solution didn't work if there were non-trivial operations involved in computing
a constant initializer which caused the 'constant unfolding' code to kick in
(addConstantReferenceConversion). Instead, this change does the 'unfolding'
later in createSpvConstantFromConstUnionArray. If a reference-type constant has
survived that long, then folding is already done, this must be a 'real' (inside
a function) use of the constant, and it should be safe to unfold and apply the
bitcast.
Allow constructors to and from references to be constant folded. Section 4.3.3
says constructors whose arguments are all constant expressions must fold.
Disallow 'const' on buffer reference types. It is not a 'non-void transparent
basic data type' (it is not considered 'basic').
Handle buffer reference constants (which can be assigned to a non-const reference,
or can be further folded to another type of constant) by converting to
'constructor(uint64_t constant)' in addConversion.
Disallow == and != operators on reference types.
