[lldb] Convert misc. StringConvert uses

Replace misc. StringConvert uses with llvm::to_integer()
and llvm::to_float(), except for cases where further refactoring is
planned.  The purpose of this change is to eliminate the StringConvert
API that is duplicate to LLVM, and less correct in behavior at the same
time.

Differential Revision: https://reviews.llvm.org/D110447