[clang] Reland "[Modules] Record whether VarDecl initializers contain side effects" (PR #145447)

[Henrik G. Olsson via cfe-commits]

hnrklssn wrote:

> What is the difference between this one and the previous PR?

The second commit, where I prevent constant evaluation if the expression type is null

https://github.com/llvm/llvm-project/pull/145447