Use FrozenStringLiteralComment instead of flag.

Unfortunately we cannot use `--frozen-string-literal` with Ruby 2.3 due
to https://bugs.ruby-lang.org/issues/12031 and our usage of ERB.

Library/.rubocop.yml

@@ -93,8 +93,7 @@ Style/ConditionalAssignment:
 Style/Documentation:
  Enabled: false
 
-# we want to add this slowly and manually
-# TODO: add to more files
+# don't want this for formulae but re-enabled for Library/Homebrew
 Style/FrozenStringLiteralComment:
   Enabled: false
 

Library/Homebrew/.rubocop.yml

@@ -117,6 +117,11 @@ Style/DocumentationMethod:
   Include:
     - 'Library/Homebrew/formula.rb'
 
+# don't want this for formulae but re-enabled for Library/Homebrew
+Style/FrozenStringLiteralComment:
+  Enabled: true
+  EnforcedStyle: always
+
 # so many of these in formulae but none in here
 Style/GuardClause:
   Enabled: true

Library/Homebrew/brew.sh

@@ -348,9 +348,6 @@ else
   RUBY_DISABLE_OPTIONS="--disable=gems,rubyopt"
 fi
 
-# Don't set this for anyone (yet)
-unset HOMEBREW_FROZEN_STRING_LITERAL
-
 if [[ -z "$HOMEBREW_RUBY_WARNINGS" ]]
 then
   export HOMEBREW_RUBY_WARNINGS="-W0"
@@ -502,5 +499,5 @@ else
 
   # Unshift command back into argument list (unless argument list was empty).
   [[ "$HOMEBREW_ARG_COUNT" -gt 0 ]] && set -- "$HOMEBREW_COMMAND" "$@"
-  { update-preinstall "$@"; exec "$HOMEBREW_RUBY_PATH" $HOMEBREW_FROZEN_STRING_LITERAL $HOMEBREW_RUBY_WARNINGS "$RUBY_DISABLE_OPTIONS" "$HOMEBREW_LIBRARY/Homebrew/brew.rb" "$@"; }
+  { update-preinstall "$@"; exec "$HOMEBREW_RUBY_PATH" $HOMEBREW_RUBY_WARNINGS "$RUBY_DISABLE_OPTIONS" "$HOMEBREW_LIBRARY/Homebrew/brew.rb" "$@"; }
 fi