[SymbolSize] Improve the performance of SymbolSize computation

The current algorithm to compute the symbol size is quadratic if there
are lots of symbols sharing the same addresses. This happens in a debug
build when lots of debug symbols get emitted in the symtab.

This patch improves the performance like `llvm-symbolizer` that relies
on the symbol size computation. Symbolizing a release+assert clang with
DebugInfo sees significant improvements from 3:40min to less than 1s.

Reviewed By: pete, mehdi_amini, arsenm, MaskRay

Differential Revision: https://reviews.llvm.org/D156603