Fix #4946 --- Update FSC and FSI to recognize upto .net framework 472 - #6005

Failing codegen test (System.ValueTuple now in mscorlib)

2018-12-13T06:24:25.1698042Z +++ CodeGen\EmittedIL\Tuples (ValueTupleAliasConstructor.fs -) +++

Also FCS is failing to build due to some undefined symbols

@KevinRansom As we discussed I think we can move FCS to .NET 4.7.2 for dev 16.0 timeframe

@dsyme Is net472 really needed?
We still target net461. I'll try to figure out if we're going to move anytime soon.

@dsyme Is net472 really needed? We still target net461. I'll try to figure out if we're going to move anytime soon.

Thanks, this is useful feedback.

My understanding is dev16.0 will take .NET 4.7.2 as a base but others can confirm.

@KevinRansom If we can get this PR in with FCS still target net461 that would be great.

I've already done something similar when we ported the dev16.0 branch to net472 in #5719.

Okay so I updated this so that:

1. Versions below DotNET 4.5 are no longer recognized
2. It searches for reference assembly profiles and selects the highest version.
3. If No reference assemblies are installed it uses the highest framework.