Avoid triple corruption while merging core info

Summary:
This patch fixes a bug where when target triple created from elf information
is arm-*-linux-eabihf and platform triple is armv8l-*-linux-gnueabihf. Merging
both triple results in armv8l--unknown-unknown.

This happens because we order a triple update while calling CoreUpdated and
CoreUpdated creates a new triple with no vendor or environment information.

Making sure we do not update triple and just update to more specific core
fixes the issue.

Reviewers: labath, jasonmolenda, clayborg

Reviewed By: jasonmolenda

Subscribers: jankratochvil, kristof.beyls, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D70155