Try once more to ensure constant initializaton of ManagedStatics

Try once more to ensure constant initializaton of ManagedStatics

First, use the old style of linker initialization for MSVC 2019 in
addition to 2017. MSVC 2019 emits a dynamic initializer for
ManagedStatic when compiled in debug mode, and according to zturner,
also sometimes in release mode. I wasn't able to reproduce that, but it
seems best to stick with the old code that works.

When clang is using the MSVC STL, we have to give ManagedStatic a
constexpr constructor that fully zero initializes all fields, otherwise
it emits a dynamic initializer. The MSVC STL implementation of
std::atomic has a non-trivial (but constexpr) default constructor that
zero initializes the atomic value. Because one of the fields has a
non-trivial constructor, ManagedStatic ends up with a non-trivial ctor.
The ctor is not constexpr, so clang ends up emitting a dynamic
initializer, even though it simply does zero initialization. To make it
constexpr, we must initialize all fields of the ManagedStatic.

However, while the constructor that takes a pointer is marked constexpr,
clang says it does not evaluate to a constant because it contains a cast
from a pointer to an integer. I filed this as:
https://developercommunity.visualstudio.com/content/problem/545566/stdatomic-value-constructor-is-not-actually-conste.html

Once we do that, we can add back the
LLVM_REQUIRE_CONSTANT_INITIALIZATION marker, and so far as I'm aware it
compiles successfully on all supported targets.

llvm-svn: 359135