[LLDB] Don't print register fields when asked for a specific format

Previously if a register had fields we would always print them after the
value if the register was asked for by name.
```
(lldb) register read MDCR_EL3
MDCR_EL3 = 0x00000000
         = {
             ETBAD = 0
<...>
             RLTE = 0
           }
```
This can be quite annoying if there are a whole lot of fields but you
want to see the register in a specific format.
```
(lldb) register read MDCR_EL3 -f i
MDCR_EL3 = 0x00000000   unknown     udf    #0x0
         = {
             ETBAD = 0
<...lots of fields...>
```
Since it pushes the interesting bit far up the terminal. To solve this,
don't print fields if the user passes --format. If they're doing that
then I think it's reasonable to assume they know what they want and only
want to see that output.

This also gives users a way to silence fields, but not change the format.
By doing `register read foo -f x`. In case they are not useful or perhaps
they are trying to work around a crash.

I have customised the help text for --format for register read to explain this:
```
-f <format> ( --format <format> )
     Specify a format to be used for display. If this is set, register fields will not be dispayed.
```

Reviewed By: jasonmolenda

Differential Revision: https://reviews.llvm.org/D148790

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