description cache invalid if updated w/o EVAL_ALL

When updating description cache, if eval_all not set, clear cache rather
than leaving it out of date.

This fixes an issue where, if a user sets `--eval-all` on the command
line to run description searches, but HOMEBREW_EVAL_ALL isn't set in the
environment the cache is never updated.

Library/Homebrew/cache_store.rb

@@ -68,6 +68,14 @@ class CacheStoreDatabase
     db.delete(key)
   end
 
+  # Deletes all content from the underlying database (if it already exists).
+  def clear!
+    return unless created?
+
+    dirty!
+    db.clear
+  end
+
   # Closes the underlying database (if it is created and open).
   def write_if_dirty!
     return unless dirty?

Library/Homebrew/description_cache_store.rb

@@ -41,7 +41,10 @@ class DescriptionCacheStore < CacheStore
   # @param report [Report] an update report generated by cmd/update.rb
   # @return [nil]
   def update_from_report!(report)
-    return unless Homebrew::EnvConfig.eval_all?
+    unless Homebrew::EnvConfig.eval_all?
+      database.clear!
+      return
+    end
     return populate_if_empty! if database.empty?
     return if report.empty?
 
@@ -60,7 +63,10 @@ class DescriptionCacheStore < CacheStore
   # @param formula_names [Array] the formulae to update
   # @return [nil]
   def update_from_formula_names!(formula_names)
-    return unless Homebrew::EnvConfig.eval_all?
+    unless Homebrew::EnvConfig.eval_all?
+      database.clear!
+      return
+    end
     return populate_if_empty! if database.empty?
 
     formula_names.each do |name|
@@ -108,7 +114,10 @@ class CaskDescriptionCacheStore < DescriptionCacheStore
   # @param report [Report] an update report generated by cmd/update.rb
   # @return [nil]
   def update_from_report!(report)
-    return unless Homebrew::EnvConfig.eval_all?
+    unless Homebrew::EnvConfig.eval_all?
+      database.clear!
+      return
+    end
     return populate_if_empty! if database.empty?
     return if report.empty?
 
@@ -124,7 +133,10 @@ class CaskDescriptionCacheStore < DescriptionCacheStore
   # @param cask_tokens [Array] the casks to update
   # @return [nil]
   def update_from_cask_tokens!(cask_tokens)
-    return unless Homebrew::EnvConfig.eval_all?
+    unless Homebrew::EnvConfig.eval_all?
+      database.clear!
+      return
+    end
     return populate_if_empty! if database.empty?
 
     cask_tokens.each do |token|