Only refer to LLVM symbol table in calls to Symbol#to_s

[HertzDevil]

The compiler places the string contents of all symbols defined in the source code into a special :symbol_table LLVM global variable in the main LLVM module:

@":symbol_table" = global [24 x ptr] [
  ptr @"'general'", ptr @"'no_error'", ptr @"'gc'", ptr @"'sequentially_consis...'",
  ptr @"'monotonic'", ptr @"'acquire'", ptr @"'evloop'", ptr @"'xchg'", ptr @"'release'",
  ptr @"'acquire_release'", ptr @"'io_write'", ptr @"'sched'", ptr @"'io_read'",
  ptr @"'skip'", ptr @"'none'", ptr @"'active'", ptr @"'done'", ptr @"'unchecked'",
  ptr @"'sleep'", ptr @"'relaxed'", ptr @"'add'", ptr @"'sub'", ptr @"'file'", ptr @"'target'"
]

Every LLVM module, without exception, also declares it:

@":symbol_table" = external global [24 x ptr]

Since the total number of symbols is part of the variable type, adding or removing any symbol invalidates all object files in the Crystal cache, even though only Symbol#to_s, or rather @[Primitive(:symbol_to_s)], has access to this variable.

This PR removes this declaration except where the primitive accesses it. Changes to the symbol table will only affect LLVM modules that call Symbol#to_s (the primitive body itself is inlined). Building an empty file, replacing it with x = :abcde, then rebuilding the same file will now give:

Codegen (bc+obj):
 - 315/317 .o files were reused

These modules were not reused:
 - _main (_main.o0.bc)
 - Crystal (C-rystal.o0.bc)

(Crystal is also invalidated because the return type of Crystal.main is now Symbol instead of Nil; appending nil to the file makes this module reusable.)

Note that it is still possible to invalidate large portions of the cache from the addition or removal of symbols, because their indices are allocated sequentially, and inlined on every use. Something like #15485 but for Symbol values would solve that.