Ensure that libSupport does not carry any static global initializer.
libSupport can be embedded in use cases where we don't want to load all
cl::opt unless we want to parse the command line.
ManagedStatic can be used to enable lazy-initialization of globals.
The -Werror=global-constructors is only added on platform that have
support for the flag and for which std::mutex does not have a global
destructor. This is ensured by having CMake trying to compile a file
with a global mutex before adding the flag to libSupport.
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