This node represents the "Canonical Frame Address" of the current frame,
and is used by various DWARF expressions to express adresses of various
objects relative to the frame.
The motivation for this is the ability to translate breakpad register
unwind rules (strings like: ".cfa 8 - ^") back into dwarf.
This patch introduces the new node type, and teaches the dwarf codegen
how to handle it. A slight complication here is that the value of CFA is
provided differently depending on the context where the dwarf expression
is used. In unwind rules, it is made available implicitly via the
initial dwarf stack. It variable location expressions, it needs to be
referred to explicitly via a special opcode.
As currently, the only usage of this is in the unwind rules, I made the
dwarf generator use this convention, but this can be easily made
configurable, if necessary. The implementation does this via keeping
track of the DWARF stack depth an any given point, and then copying the
CFA value from the bottom of the stack via the DW_OP_pick opcode. This
could be made more efficient for simple expressions, but here I chose to
start with the most general implementation possible.