[ELF] - Change the way of sorting local symbols.

rLLD329787 added the stable sorting to SymbolTableBaseSection::postThunkContents.

I profiled the Mozilla (response-O0.txt) from lld-speed-test package and found
std::stable_sort is showing up in profile results and consuming the 3.1% of the total
CPU time in the RelWithDebug build. Total time of postThunkContents is 3.54%, 238ms.

This change reduces postTimeContents time to 50ms, making it to take 0.73% of Total CPU time.

So, instead of sorting the local part I suggest to just rebuild it.
That is what this patch does.

Differential revision: https://reviews.llvm.org/D45519

llvm-svn: 335583