[ELF] Rename R_TLS to R_TPREL and R_NEG_TLS to R_TPREL_NEG. NFC

The scope of R_TLS (TP offset relocation types (TPREL/TPOFF) used for the
local-exec TLS model) is actually narrower than its name may imply. R_TLS_NEG
is only used by Solaris R_386_TLS_LE_32.

Rename them so that they will be less confusing.

Reviewed By: grimar, psmith, rprichard

Differential Revision: https://reviews.llvm.org/D93467