[lld/pdb] Speed up construction of publics & globals addr map.

computeAddrMap function calls std::stable_sort with a comparison
function that computes deserialized symbols every time its called.
In the result deserializeAs<PublicSym32> is called 20-30 times per
symbol. It's much faster to calculate it beforehand and pass a
pointer to it to the comparison function.

Patch by Alex Telishev
Differential Revision: https://reviews.llvm.org/D36941

llvm-svn: 311373