[lldb] [Host] Refactor Socket::DecodeHostAndPort() to use LLVM API

Refactor Socket::DecodeHostAndPort() to use LLVM API over redundant
LLDB API.  In particular, this means llvm::Regex, llvm::Error return
type and llvm::to_integer().

While at it, change the port type from int32_t to uint16_t.  The method
never returns any value outside this range, and using the correct type
allows us to rely on getAsInteger()'s implicit overflow check.

Differential Revision: https://reviews.llvm.org/D110391