Cat() not returning a literal for literal inputs

When `Cat()` (or `##`) is called on literal nodes, the result is not a literal node anymore:

```scala
val hwInfo = Cat(numCores.U(4.W), buildNum.U(4.W))
assert(hwInfo.isLit()) //fails
```

This is at least surprising to a Chisel newbie like me. I known that the Cat function merely generated the corresponding FIRRTL statement, and constant folding is done later in the process, so this might be out-of-scope. On the other hand, printing out the value of literals is very handy for the automatic creating of documentation (register maps in my case).

One might consider adding a `LitCat()` function, which could than be even called outside of a Module context, which would be very handy for testing.

**Type of issue**: feature request

**Impact**: no functional change

**Development Phase**: request

**Please tell us about your environment:**

- version: `3.3.2`

**What is the use case for changing the behavior?**
Automatic generation of documentation, building literals in test code (in the `LitCat` case)

Start by examining the Cat() and ## entry points and how they handle literal nodes before FIRRTL constant folding. Compare the current behavior with the proposed LitCat() use case. Done means literal inputs produce a literal result, with behavior suitable for documentation generation and test code.

Written by the indexing model from the issue text.