See code commentary for an explanation.
Add an additional constructor to allow `log_linux.cc` to compile with
GPR_PTHREAD_TLS. Without it:
```
../../third_party/grpc/src/core/lib/gpr/log_linux.cc:78:33: error: no viable conversion from 'int' to 'grpc_core::PthreadTlsImpl<long>'
static GPR_THREAD_LOCAL(long) tid = 0;
^ ~
../../third_party/grpc/src/core/lib/gpr/tls.h:64:3: note: candidate constructor not viable: no known conversion from 'int' to 'const grpc_core::PthreadTlsImpl<long> &' for 1st argument
PthreadTlsImpl(const PthreadTlsImpl&) = delete;
^
1 error generated.
```
* Implement type safety for TLS
This is mostly free when compiler support is available, but requires
careful templating when implemented using pthread.
Significantly slimmed the tls.h interface; it now only defines the "TLS
keyword" for each supported compiler, delegating enforcement of correct
usage (i.e. must be static) to the compiler itself.
Implemented implicit conversion for the pthread wrapper so it can be
used (mostly) the same as native support. Notable exception to this is
that static_cast<void*> is needed when printing a pointer stored in TLS
as %p.
* Use GPR_THREAD_LOCAL macros consistently
Craig Tiller