Avoid std::string with reentrant_allocator in ElfCache

Summary:
The implementation of `std::basic_string` with the legacy ABI and with `_GLIBCXX_FULLY_DYNAMIC_STRING`, as appears by default on CentOS 7, incorrectly requires the provided allocator type to have a default constructor. The `Allocator` concept does not require this and many allocator types with runtime state shared between copies will not provide this.

Switch to `folly::fbstring` in `ElfCache` to sidestep this problem.

Fixes: https://github.com/facebook/folly/issues/1344.

Reviewed By: yfeldblum

Differential Revision: D22970418

fbshipit-source-id: 1e188d668991d25f53da5702b31f4a7a7ec7d7e8

folly/experimental/symbolizer/ElfCache.h

@@ -24,6 +24,7 @@
 
 #include <boost/intrusive/avl_set.hpp>
 
+#include <folly/FBString.h>
 #include <folly/Range.h>
 #include <folly/experimental/symbolizer/Elf.h>
 #include <folly/hash/Hash.h>
@@ -47,10 +48,8 @@ class SignalSafeElfCache : public ElfCacheBase {
  public:
   std::shared_ptr<ElfFile> getFile(StringPiece path) override;
 
-  using Path = std::basic_string< //
-      char,
-      std::char_traits<char>,
-      reentrant_allocator<char>>;
+  using Path =
+      basic_fbstring<char, std::char_traits<char>, reentrant_allocator<char>>;
 
   struct Entry : boost::intrusive::avl_set_base_hook<> {
     Path path;