Parse and format default function arguments

[osandov]

d35243b added a representation for default function arguments in the drgn API, but we don't actually parse them from DWARF. As of this writing, neither GCC nor Clang actually emits the required DWARF, but we should prepare for it if we can.

Parsing

Section 4.1 in the DWARF 5 spec defines how default arguments are represented:

A DW_AT_default_value attribute for a formal parameter entry. The value of this attribute may be a constant, or a reference to the debugging information entry for a variable, or a reference to a debugging information entry containing a DWARF procedure. If the attribute form is of class constant, that constant is interpreted as a value whose type is the same as the type of the formal parameter. If the attribute form is of class reference, and the referenced entry is for a variable, the default value of the parameter is the value of the referenced variable. If the reference value is 0, no default value has been specified. Otherwise, the attribute represents an implicit DW_OP_call_ref to the referenced debugging information entry, and the default value of the parameter is the value returned by that DWARF procedure, interpreted as a value of the type of the formal parameter.

For a constant form there is no way to express the absence of a default value.

The constant form is easy. The variable reference form is also relatively easy, except it doesn't specify how the type of the variable is related to the type of the parameter: are they supposed to be the same type, or are we supposed to convert it to the parameter type if the variable type differs? It's hard to tell without real producers. A reference to a DWARF procedure is obviously going to require DWARF expression support.

Formatting

We also currently don't include the default arguments when printing a function type. We can probably reuse the struct/array initializer formatting code to format parameters with default arguments.