Change grammar underproduction to overproduction.

The old grammar didn't allow things like f(&x.y) which are legal in
Swift.  The new grammar allows a bunch of stuff -- literals, closures,
and conditional expressions -- but that's a better problem to have.

See also this PR, where we're discussing a more specific fix:

    #194

TSPL.docc/ReferenceManual/Expressions.md

@@ -61,7 +61,7 @@ as described in <doc:Expressions#Implicit-Conversion-to-a-Pointer-Type>.
 
 > Grammar of an in-out expression:
 >
-> *in-out-expression* → **`&`** *identifier*
+> *in-out-expression* → **`&`** *primary-expression*
 
 ### Try Operator
 

TSPL.docc/ReferenceManual/SummaryOfTheGrammar.md

@@ -323,7 +323,7 @@ make the same change here also.
 
 > Grammar of an in-out expression:
 >
-> *in-out-expression* → **`&`** *identifier*
+> *in-out-expression* → **`&`** *primary-expression*
 
 > Grammar of a try expression:
 >