folly/Traits.h: fix a deep problem with FBVector

folly/Traits.h: fix a deep problem with FBVector

Summary:
This started when many fbvector-related tests began failing with reports of
ASAN-detected heap abuse (usually invalid free of an address 16 bytes
into an already-freed buffer).

It was very specific, though. The bug struck only when:
  - gcc-5-based platform
  - compiling with clang (~3.8) and GCC5's libstdc++, not gcc
  - strings were short enough to reside within an in-situ std::string

Why? because FBVector.h:1588's conditional:

  if (folly::IsRelocatable<T>::value && usingStdAllocator::value) {

was true for clang (erroneously), but false for gcc. The difference
was in `IsRelocatable<std::string>::value`.  However, manual tests showed that
each component from the definition of `IsRelocatable` were the same for both
gcc and clang. Then Jay Feldblum realized that we'd probably specialized
that trait for `std::string`, and sure enough, I found the culprit:

```
FOLLY_ASSUME_FBVECTOR_COMPATIBLE_3(std::basic_string)
```

Reviewed By: pixelb

Differential Revision: D4414159

fbshipit-source-id: 60e3fb4b096ec77cbd2b48c561e5c6ec8f128fff