[MC] Pass the symbol rather than its name to onSymbolStart()

Summary: This allows targets to also consider the symbol's type and/or address if needed.

Reviewers: scott.linder, jhenderson, MaskRay, aardappel

Reviewed By: scott.linder, MaskRay

Subscribers: dschuff, sbc100, jgravelle-google, hiraditya, aheejin, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82090