MS ABI: Implement driver-level support for thread-safe statics

MS ABI: Implement driver-level support for thread-safe statics

Decide whether or not to use thread-safe statics depending on whether or
not we have an explicit request from the driver.  If we don't have an
explicit request, infer which behavior to use depending on the
compatibility version we are targeting.

N.B. CodeGen support is still ongoing.

llvm-svn: 232906