[llvm-profgen] Clean up unnecessary memory reservations between phases.

Cleaning up data structures that are not used after a certain point. This further brings down peak memory usage by 15% for a large benchmark.

Before:
   note: Before parsePerfTraces
   note: VM: 40.73 GB   RSS: 39.18 GB
   note: Before parseAndAggregateTrace
   note: VM: 40.73 GB   RSS: 39.18 GB
   note: After parseAndAggregateTrace
   note: VM: 88.93 GB   RSS: 87.97 GB
   note: Before generateUnsymbolizedProfile
   note: VM: 88.95 GB   RSS: 87.99 GB
   note: After generateUnsymbolizedProfile
   note: VM: 93.50 GB   RSS: 92.53 GB
   note: After computeSizeForProfiledFunctions
   note: VM: 101.13 GB   RSS: 99.36 GB
   note: After generateProbeBasedProfile
   note: VM: 215.61 GB   RSS: 210.88 GB
   note: After postProcessProfiles
   note: VM: 237.48 GB   RSS: 212.50 GB

After:
   note: Before parsePerfTraces
   note: VM: 40.73 GB   RSS: 39.18 GB
   note: Before parseAndAggregateTrace
   note: VM: 40.73 GB   RSS: 39.18 GB
   note: After parseAndAggregateTrace
   note: VM: 88.93 GB   RSS: 87.96 GB
   note: Before generateUnsymbolizedProfile
   note: VM: 88.95 GB   RSS: 87.97 GB
   note: After generateUnsymbolizedProfile
   note: VM: 93.50 GB   RSS: 92.51 GB
   note: After computeSizeForProfiledFunctions
   note: VM: 93.50 GB   RSS: 92.53 GB
   note: After generateProbeBasedProfile
   note: VM: 164.87 GB   RSS: 163.55 GB
   note: After postProcessProfiles
   note: VM: 182.28 GB   RSS: 179.43 GB

Reviewed By: wenlei, wlei

Differential Revision: https://reviews.llvm.org/D118677