[vm,dyn_modules] Initial handling of breakpoints in the interpreter.

Adds a set of new VMInternal_Breakpoint instructions, one for each
possible instruction size, and adds a bytecode_ and saved_opcode_
field to code breakpoints.

When enabling a breakpoint, the original opcode of the instruction is
replaced with the same-sized VmInternal_Breakpoint opcode and stored in
the saved_opcode_ field of the CodeBreakpoint. When disabling it, the
original opcode is replaced.

New labels are added to the dispatch loop for single stepping
purposes. Both the computed goto and switch dispatch versions
of the dispatch loop are appropriately altered to dispatch to
the single stepping labels instead of the original ones if single
stepping is currently enabled.

Fix up more parts of the debugger that assumed functions had Code
objects to handle functions with Bytecode objects as well. In
particular, instead of using the PcDescriptors to find safepoint
source locations in Bytecode objects, the source positions information
is used instead (since the PcDescriptors for Bytecode objects only
stores information about the start and end of try blocks at the moment).

Todo (from looking at the remaining failing tests):
* Handle async jumps.
* Handle coverage information.

TEST=now-passing tests from pkg/vm_service like
     pkg/vm_service/test/break_on_function_test

Change-Id: Icbd4b818e00508d9a4e74c81520aad2363b26d41
Cq-Include-Trybots: luci.dart.try:vm-dyn-linux-debug-x64-try,vm-aot-dyn-linux-debug-x64-try,vm-aot-dyn-linux-product-x64-try,vm-dyn-mac-debug-arm64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/444880
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Tess Strickland <sstrickl@google.com>