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