[ThinLTO/gold] Perform cache pruning when cache directory specified

[tejohnson]

Summary

As pointed out in the review for D37993, for consistency with other
linkers, gold plugin should perform cache pruning whenever there is a
cache directory specified, which will use the default cache policy.

[tejohnson]

ping

[pcc]

LGTM

llvm/trunk/test/tools/gold/X86/cache.ll

 ; Verify that enabling caching is ignoring module when we emit them without hash.
 ; RUN: opt -module-summary %s -o %t.o
 ; RUN: opt -module-summary %p/Inputs/cache.ll -o %t2.o
 
 ; RUN: rm -Rf %t.cache
 ; RUN: %gold -m elf_x86_64 -plugin %llvmshlibdir/LLVMgold%shlibext \
 ; RUN:     --plugin-opt=thinlto \
 ; RUN:     --plugin-opt=cache-dir=%t.cache \
 ; RUN:     -o %t3.o %t2.o %t.o
 
-; RUN: ls %t.cache | count 0
+; We should just get the timestamp file
+; RUN: ls %t.cache | count 1
 
 
 ; Verify that enabling caching is working with module with hash.
 
 ; RUN: opt -module-hash -module-summary %s -o %t.o
 ; RUN: opt -module-hash -module-summary %p/Inputs/cache.ll -o %t2.o
 
 ; RUN: rm -Rf %t.cache
 ; RUN: %gold -m elf_x86_64 -plugin %llvmshlibdir/LLVMgold%shlibext \
 ; RUN:     --plugin-opt=thinlto \
 ; RUN:     --plugin-opt=cache-dir=%t.cache \
 ; RUN:     -o %t3.o %t2.o %t.o
 
-; RUN: ls %t.cache | count 2
+; Two cached objects, plus a timestamp file
+; RUN: ls %t.cache | count 3
 
 
 ; Create two files that would be removed by cache pruning due to age.
 ; We should only remove files matching the pattern "llvmcache-*".
 
 ; RUN: touch -t 197001011200 %t.cache/llvmcache-foo %t.cache/foo
 ; RUN: %gold -m elf_x86_64 -plugin %llvmshlibdir/LLVMgold%shlibext \
 ; RUN:     --plugin-opt=thinlto \

llvm/trunk/tools/gold/gold-plugin.cpp

static ld_plugin_status cleanup_hook(void) {
   for (std::string &Name : Cleanup) {
     std::error_code EC = sys::fs::remove(Name);
     if (EC)
       message(LDPL_ERROR, "Failed to delete '%s': %s", Name.c_str(),
               EC.message().c_str());
   }
 
   // Prune cache
-  if (!options::cache_policy.empty()) {
+  if (!options::cache_dir.empty()) {
     CachePruningPolicy policy = check(parseCachePruningPolicy(options::cache_policy));
     pruneCache(options::cache_dir, policy);
   }
 
   return LDPS_OK;
 }