No way to get real x86 register "sp" from an SBValue representing a register set

No intended way at least.

Set 0 is general purpose registers:
```
(lldb) script
Python Interactive Interpreter. To exit, type 'quit()', 'exit()' or Ctrl-D.
>>> lldb.frame.GetRegisters()[0]
General Purpose Registers = {
rax = 0x0000555555555129
rbx = 0x0000000000000000
rcx = 0x0000555555557df8
rdx = 0x00007fffffffdbd8
rdi = 0x0000000000000001
rsi = 0x00007fffffffdbc8
rbp = 0x00007fffffffdab0
rsp = 0x00007fffffffdab0
}
```
"rsp" is at index 7, but so is "sp". Where "sp" means the ABI register name, which due to code in `RegisterContext::GetRegisterInfoByName` is converted to "rsp" before lookup.
```
>>> lldb.frame.GetRegisters()[0].GetIndexOfChildWithName("rsp")
7
>>> lldb.frame.GetRegisters()[0].GetIndexOfChildWithName("sp")
7
```
The "real" "sp" is at a different index:
```
>>> lldb.frame.GetRegisters()[0].GetChildAtIndex(49)
(unsigned short) sp = 0xdab0
```

You can work around this by taking advantage of the fact that the name conversion is not case sensitive:
```
>>> lldb.frame.GetRegisters()[0].GetIndexOfChildWithName("sP")
49
```
`sp` is converted to `rsp`, but `sP` is not, is then lower cased, and `sp` is found.

But this in itself is a bug.

Reproduce the issue with the LLDB Python commands in the report, then inspect RegisterContext::GetRegisterInfoByName and the register-set child-name lookup it affects. Done means the ABI name "sp" and the real register named "sp" can be addressed distinctly without relying on case variation, with regression coverage for the reported lookups.

Written by the indexing model from the issue text.