[ELF] Remove StringRefZ

StringRefZ does not improve performance. Non-local symbols always have eagerly
computed nameSize. Most local symbols's lengths will be updated in either:

* shouldKeepInSymtab
* SymbolTableBaseSection::addSymbol

Its benefit is offsetted by strlen in every call site (sums up to 5KiB code in a
release x86-64 build), so using StringRefZ may be slower.

In a -s link (uncommon) there is minor speedup, like ~0.3% for clang and chrome.

Reviewed By: alexander-shaposhnikov

Differential Revision: https://reviews.llvm.org/D117644