core: EvalWriteOutput discard unknown values before writing state
core: EvalWriteOutput discard unknown values before writing state

The state only deals in wholly-known values, so here we null out any
unknowns for storage in state. This is okay because we subsequently write
the original, possibly-unknown value into the plan and the expression
evaluator will prefer to use this if present, allowing the unknown values
to properly propagate into other expressions in the calling module.

mattray/terraform,hashicorp/terraform,apparentlymart/terraform,brandontosch/terraform,pdecat/terraform,luis-silva/terraform,drebes/terraform,gechr/terraform,VladRassokhin/terraform,brandontosch/terraform,tpounds/terraform,mattray/terraform,mattray/terraform,apparentlymart/terraform,apparentlymart/terraform,tommynsong/terraform,pradeepbhadani/terraform,pdecat/terraform,pryorda/terraform,hashicorp/terraform,tommynsong/terraform,nlamirault/terraform,heimweh/terraform,apparentlymart/terraform,iljaskevic/terraform,ewbankkit/terraform,nlamirault/terraform,VladRassokhin/terraform,brandontosch/terraform,VladRassokhin/terraform,iljaskevic/terraform,mattray/terraform,drebes/terraform,youhong316/terraform,rnaveiras/terraform,luis-silva/terraform,heimweh/terraform,youhong316/terraform,brandontosch/terraform,heimweh/terraform,brandontosch/terraform,nlamirault/terraform,gechr/terraform,iljaskevic/terraform,drebes/terraform,pradeepbhadani/terraform,heimweh/terraform,ewbankkit/terraform,tpounds/terraform,tommynsong/terraform,tpounds/terraform,iljaskevic/terraform,luis-silva/terraform,rnaveiras/terraform,pradeepbhadani/terraform,tpounds/terraform,pdecat/terraform,ewbankkit/terraform,apparentlymart/terraform,luis-silva/terraform,pryorda/terraform,hashicorp/terraform,VladRassokhin/terraform,gechr/terraform,pdecat/terraform,ewbankkit/terraform,drebes/terraform,youhong316/terraform,heimweh/terraform,pryorda/terraform,youhong316/terraform,rnaveiras/terraform