[PGO] Add hash to name mapping in InstrProfSymtab

Creator and lookup interfaces are added to this symtab class.
The new interfaces will be used by InstrProf Readers and writer.

A unit test is also added for the new APIs.




git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256092 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed