[ELF] Allow local symbols to be added after global symbols

    
This change moves the SymbolBodies with isLocal() == true before the global
symbols then calculating NumLocals rather than assuming all locals are
added before globals and the first NumLocals have isLocal() == true. This
permits Thunks to be moved after the pass that adds global symbols from
synthetics to the symbol table.

Differential revision: https://reviews.llvm.org/D30085

llvm-svn: 295650