[ELF] Add makeThreadLocal/makeThreadLocalN and remove InputFile::localSymStorage

makeThreadLocal/makeThreadLocalN are moved from D130810 ([ELF] Parallelize input
section initialization) here to make D130810 more focused on the refactor:

* COFF has some needs for multiple linker contexts. D108850 partially removed
  global states from lldCommon but left the global variable `lctx`.
* To the best of my knowledge, all multiple-linker-context feature requests to
  ELF are more from user convenience, with no very strong argument.
* In practice, ELF port is very difficult to remove global states without
  introducing significant performance regression/hurting code readability.
* Per-thread allocators from D122922/D123879 are too expensive and will not
  really benefit ELF.

This patch adds a simple thread_local based makeThreadLocal to
lld/Common/Memory.h. It will enable further optimization in ELF.