Revert rL357655 and rL357656 from llvm/trunk:

Fix minor innaccuracy in previous comment on ManagedStaticBase
........
Make ManagedStatic constexpr constructible

Apparently it needs member initializers so that it can be constructed in
a constexpr context. I explained my investigation of this in PR41367.
........
Causes vs2017 debug llvm-tblgen to fail with "Unknown command line argument" errors - similar to the vs2019 error discussed on PR41367 without the patch....

llvm-svn: 357685