[BOLT] Section-handling refactoring/overhaul

Simplify the logic of handling sections in BOLT. This change brings more
direct and predictable mapping of BinarySection instances to sections in
the input and output files.

* Only sections from the input binary will have a non-null SectionRef.
  When a new section is created as a copy of the input section,
  its SectionRef is reset to null.

* RewriteInstance::getOutputSectionName() is removed as the section name
  in the output file is now defined by BinarySection::getOutputName().

* Querying BinaryContext for sections by name uses their original name.
  E.g., getUniqueSectionByName(".rodata") will return the original
  section even if the new .rodata section was created.

* Input file sections (with relocations applied) are emitted via MC with
  ".bolt.org" prefix. However, their name in the output binary is
  unchanged unless a new section with the same name is created.

* New sections are emitted internally with ".bolt.new" prefix if there's
  a name conflict with an input file section. Their original name is
  preserved in the output file.

* Section header string table is properly populated with section names
  that are actually used. Previously we used to include discarded
  section names as well.

* Fix the problem when dynamic relocations were propagated to a new
  section with a name that matched a section in the input binary.
  E.g., the new .rodata with jump tables had dynamic relocations from
  the original .rodata.

Reviewed By: rafauler

Differential Revision: https://reviews.llvm.org/D135494