[Support] Replace Windows __declspec(thread) with thread_local for LLVM_THREAD_LOCAL

Windows minimum host tools version is now VS2017, which supports C++11
thread_local so use this for LLVM_THREAD_LOCAL instead of
declspec(thread). According to [1], thread_local is implemented with
declspec(thread) so this should be NFC.

[1] https://docs.microsoft.com/en-us/cpp/cpp/thread?view=vs-2017

Differential Revision: https://reviews.llvm.org/D72399