Inconsistent/Misleading error messages for illegal ReferenceTargets

**Type of issue**: Bug Report

**Please provide the steps to reproduce the problem:**
```
class Bar extends RawModule {
val vec = IO(Input(Vec(3,Bool())))
val lit = 1.U
val notLit = vec(lit)
}
class Foo extends RawModule {
val bar = Module(new Bar)
bar.notLit.toTarget
}
ChiselStage.elaborate(new Foo)
}
```
**What is the current behavior?**
```
Vectors which are indexed by literal values should successfully be converted to Reference Target *** FAILED ***
[info] chisel3.internal.ChiselException: Illegal component name: vec[UInt<1>("h01")] (note: literals are illegal)
[info]
```
**What is the expected behavior?**

I would get a Reference Target attached to the not-literal vector element

**Please tell us about your environment:**
3.5.x

**Other Information**

**What is the use case for changing the behavior?**

wanting to put reference targets on specific vector elements