[lldb][NFCI] Remove StructuredData::Dictionary::GetValueForKeyAsString...

[lldb][NFCI] Remove StructuredData::Dictionary::GetValueForKeyAsString overloads involving ConstString

In an effort to keep the ConstString StringPool small, I plan on
removing use of ConstString in StructuredData. The only class that
really uses it is StructuredData::Dictionary.

This one was fairly easy to remove, I plan on removing the others in
follow-up changes.

Differential Revision: https://reviews.llvm.org/D152597