[LLD] [COFF] Omit section symbols and IMAGE_SYM_CLASS_LABEL from the PE symbol table

The section symbols aren't of much practical use when looking at
a linked image. This shrinks one observed mingw style unstripped
binary by 14%.

IMAGE_SYM_CLASS_LABEL is in spirit the same as a temporary assembler
label that isn't emitted on the object file level at all.

Differential Revision: https://reviews.llvm.org/D113866