[Object][NFC] Don't insert string table into object members vector.

Having string table in members vector does not fit later patches in
this series. Symbol map needs to refer to objects' offsets, but string
table should not be referenced. Also for ARM64EC, the new <ECSYMBOLS>
table is inserted after string table.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D143538