simpleperf: print file path used for reading symbols.

It helps users to find out which files are used for
reporting.

Bug: http://b/29574526
Change-Id: I3d608e61c50471632c50bf6e8f6f9e45c63fc4b4
Test: run `simpleperf report --log verbose` manually.
diff --git a/simpleperf/dso.cpp b/simpleperf/dso.cpp
index 69049e2..bd6c1ba 100644
--- a/simpleperf/dso.cpp
+++ b/simpleperf/dso.cpp
@@ -265,6 +265,7 @@
 
 bool CheckReadSymbolResult(ElfStatus result, const std::string& filename) {
   if (result == ElfStatus::NO_ERROR) {
+    LOG(VERBOSE) << "Read symbols from " << filename << " successfully";
     return true;
   } else if (result == ElfStatus::NO_SYMBOL_TABLE) {
     // Lacking symbol table isn't considered as an error but worth reporting.
@@ -374,7 +375,7 @@
         std::bind(ElfFileSymbolCallback, std::placeholders::_1, this,
                   SymbolFilterForDso));
     if (result == ElfStatus::NO_ERROR) {
-      return true;
+      return CheckReadSymbolResult(result, "/usr/lib/debug" + path_);
     }
   }
   ElfStatus result = ParseSymbolsFromElfFile(