These GdbSymbol::CuVector cause memory allocation and cost lots of
memory. This patch splits off the field and add a separate CuVectors to
be more memory efficient.
For one of our large internal targets, there are 4791276 symbols and the
sum of size (eapacity) of GdbSymbol::CuVector is 19740000 (26185902).
Before: 24.820 seconds, 13.74GiB
After: 24.175 seconds, 13.21GiB
As a comparison,
/usr/bin/gold (Debian): 134.29 seconds, 12.04GiB
lld --no-gdb-index: 20.619 seconds, 9.12GiB