[ELF] - Detemplate StringTableSection.

StringTableSection was <ELFT> templated previously,

It disallow to de-template code that uses it,
for example LinkerScript<ELFT>::discard uses it as:
if (S == In<ELFT>::ShStrTab)
  error("discarding .shstrtab section is not allowed");

It seems we can try to detemplate some of synthetic sections
and somehow make them available for non-templated calls.
(move out of In<ELFT> struct probably).

Differential revision: https://reviews.llvm.org/D30933

llvm-svn: 297815