[TableGen] Use StringTable for searchable tables (#206252)
LLVM has some large searchable tables containing string pointers. This
patch changes two things:
- String references in searchable tables are now always StringTable
offsets; and all code referencing these is updated accordingly. This
often avoids relocations in the data structures, permitting their
placement in .rodata instead of .data.rel.ro and avoids dynamic
relocations.
- The lookup indicies now reference the same string table instead of
storing string pointers, permitting deduplication and also avoids
dynamic relocations.
In an all-target assert build, this changes section sizes as follows:
- .data.rel.ro: -311920
- .rodata: +227712
- .rela.dyn: -308520 (-12855 entries)
Note that due to 32-bit string offsets, most structures also got
smaller. There is room for further size reductions by using 8/16 bit
integers for string offsets where applicable and by permitting
lower-case search indices when most entries are lower-case (avoids
storing all strings twice) -- both can be done in later changes.
