[style] Consistent header guards HEIR_LIB_* vs LIB_*

[asraa]

This is just a style / formatting issue. It seems like half our headers and TD files use HEIR_LIB_* and half use LIB_*. I suspect that we had two periods of externalizing code / copybara header guard transforms and also moving the include/ dir into the lib/ dir that probably caused some confusion.

[Jeremy Kun]

Here's the macro I have in my neovim config, which creates/updates the include guard automatically. It does not include the HEIR_ prefix.

https://github.com/j2kun/nvim-config/blob/b8ef6c4bdc7425fa2db537caa4202f094cf5d1aa/lua/mappings.lua#L61-L72

local function build_include_guard()
  -- project relative filepath
  local abs_path = vim.fn.expand("%")
  local rel_path = vim.fn.fnamemodify(abs_path, ":~:.")

  -- screaming case
  local upper = string.upper(rel_path)
  -- underscore separated
  local underscored = string.gsub(upper, "[./]", "_")
  -- trailing underscore
  return underscored .. "_"
end

[asraa]

Looks like the tests/ folder is also still including THIRD_PARTY_HEIR_TESTS_ -
https://github.com/search?q=repo%3Agoogle%2Fheir%20THIRD_PARTY_HEIR_&type=code