[llvm-profdata] Drop profile symbol list during merging AutoFDO profiles.

Adding a switch to drop profile symbol list during merging AutoFDO profiles. This is needed to minimize the impact on default profiles when the profile symbol list is enabled for the source input profiles. The symbol list is quite large and could potentially slow down the compiler.

Reviewed By: davidxl, wenlei

Differential Revision: https://reviews.llvm.org/D139486