[Clang] [Sema] Fix -Wdouble-promotion in C++ list-initialization (#159992)
Resolves https://github.com/llvm/llvm-project/issues/33409.
The information `IsListInit` is already passed to function
`CheckImplicitConversion` for another use-case which makes adding a
condition for the double-promotion case simple.
Also adds tests, both for the changed list-initialization case as well
as for normal explicit casts which already would have passed before this
PR. These negative tests are added directly next to the positive tests
in `warn-double-promotion.c` or for the C++-specific cases in a new .cpp
version of that file.
llvm-project
3cc56dd8 - [Clang] [Sema] Fix -Wdouble-promotion in C++ list-initialization (#159992)
