[IR] Add missing GlobalAlias copying of ThreadLocalMode attribute

Summary:
Previously, GlobalAlias::copyAttributesFrom did not preserve ThreadLocalMode,
causing incorrect IR generation in IR linking flows. This patch pushes the code
responsible for copying this attribute from GlobalVariable::copyAttributesFrom
down to GlobalValue::copyAttributesFrom so that it is shared by GlobalAlias.
Fixes PR46297.

Reviewers: tejohnson, pcc, hans

Reviewed By: tejohnson, hans

Subscribers: hiraditya, ibookstein, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81605