[lldb] Take StringRef name in GetChildMemberWithName (NFC)

`GetChildMemberWithName` does not need a `ConstString`. This change makes the function
take a `StringRef` instead, which alleviates the need for callers to construct a
`ConstString`. I don't expect this change to improve performance, only ergonomics.

This is in support of Alex's effort to replace `ConstString` where appropriate.

There are related `ValueObject` functions that can also be changed, if this is accepted.

Differential Revision: https://reviews.llvm.org/D151615