llvm-c: use typedef for function pointers

This makes it consistent with other function pointers used in llvm-c

Differential Revision: http://llvm-reviews.chandlerc.com/D1712
<rdar://problem/14393032>

DumpValueObject() 2.0

This checkin restores pre-Xcode5 functionality to the "po" (expr -O) command:
- expr now has a new --description-verbosity (-v) argument, which takes either compact or full as a value (-v is the same as -vfull)
 When the full mode is on, "po" will show the extended output with type name, persistent variable name and value, as in
(lldb) expr -O -v -- foo
(id) $0 = 0x000000010010baf0 {
    1 = 2;
    2 = 3;
}

 When -v is omitted, or -vcompact is passed, the Xcode5-style output will be shown, as in
(lldb) expr -O -- foo
{
    1 = 2;
    2 = 3;
}

- for a non-ObjectiveC object, LLDB will still try to retrieve a summary and/or value to display
(lldb) po 5
5
-v also works in this mode
(lldb) expr -O -vfull -- 5
(int) $4 = 5

On top of that, this is a major refactoring of the ValueObject printing code. The functionality is now factored into a ValueObjectPrinter class for easier maintenance in the future
DumpValueObject() was turned into an instance method ValueObject::Dump() which simply calls through to the printer code, Dump_Impl has been removed

Test case to follow