Partial fix for PR26619.
Prior to this change, a DIGlobalVariable corresponding to a static
const was marked with an expression corresponding to its constant
value only if it is of integral type. With this change, we now do the
same if it is of __fp16, float, or double type (that is,
floating-point types that do not exceed 64 bits in size, and hence are
supported easily by the existing LLVM machinery for creating constant
expressions in debug info).