Allow enum and union literals to also create SSA values

[scottmcm]

Today, Some(x) always goes through an alloca, even in trivial cases where the niching means the constructor doesn't even change the value.

For example, https://rust.godbolt.org/z/6KG6PqoYz

pub fn demo(r: &i32) -> Option<&i32> {
    Some(r)
}

currently emits the IR

define align 4 ptr @demo(ptr align 4 %r) unnamed_addr {
start:
  %_0 = alloca [8 x i8], align 8
  store ptr %r, ptr %_0, align 8
  %0 = load ptr, ptr %_0, align 8
  ret ptr %0
}

but with this PR it becomes just

define align 4 ptr @demo(ptr align 4 %r) unnamed_addr {
start:
  ret ptr %r
}

(Of course the optimizer can clean that up, but it'd be nice if it didn't have to -- especially in debug where it doesn't run. This is like #123886, but that only handled non-simd structs -- this PR generalizes it to all non-simd ADTs.)

Doing this means handing variants other than FIRST_VARIANT, handling the active field for unions, refactoring the discriminant code so the Place and Operand parts can share the calculation, etc.

Other PRs that led up to this one:

• Change tag_field to FieldIdx in Variants::Multiple #142005
• Update InterpCx::project_field to take FieldIdx #142103
• Remove unneeded FunctionCx from some codegen methods #142324
• CodeGen: rework Aggregate implemention for rvalue_creates_operand cases #142383

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try-job: aarch64-gnu