[PGO] InstrPGO and coverage code refactoring (NFC)

Introduce a new class InstrProfSymtab to abstract
the PGO symbol table for prof and coverage reader.
The symtab is is to lookup function's PGO name
using function keys. The first user of the class
is CoverageMapping Reader. More will follow.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255862 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed