Remove disabling -Warray-bounds for GCC in FBString.h (#999)

Summary:
- Existing code disables `-Warray-bounds` warning for GCC due to a bug in
  GCC 4.9.
- As seen in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59124, the bug
  has been resolved for GCC 5.1.
- Since the bug has been resolved, we no longer need to disable the
  warning.

yfeldblum I suspect that we have to hold off on this PR until the internal use cases relying on GCC 4.9 upgrade, but I figured it is better to have some of these cleanup opportunities ready for when the time arises.
Pull Request resolved: https://github.com/facebook/folly/pull/999

Reviewed By: ot, Orvid

Differential Revision: D13729723

Pulled By: yfeldblum

fbshipit-source-id: da33704c2e2022c2dad0681e4d6152105a1cf54d

folly/FBString.h

@@ -78,10 +78,6 @@
 FOLLY_PUSH_WARNING
 // Ignore shadowing warnings within this file, so includers can use -Wshadow.
 FOLLY_GNU_DISABLE_WARNING("-Wshadow")
-// GCC 4.9 has a false positive in setSmallSize (probably
-// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59124), disable
-// compile-time array bound checking.
-FOLLY_GNU_DISABLE_WARNING("-Warray-bounds")
 
 // FBString cannot use throw when replacing std::string, though it may still
 // use folly::throw_exception