x/tools/internal/gcimporter: update iexport.go to support generics

[findleyr]

CL 349949 updates x/tools/internal/gcimporter/iimport.go to support generics, by porting over changes from the standard library go/internal/gcimporter package. It will take longer to port iexport.go, as it must be translated from cmd/compile/internal/typecheck/iexport.go. In the meantime (~days, I expect), I think we should skip x/tools/go/internal/gcimporter.TestIExportData_stdlib on Go 1.18, in order to avoid failures on the x/tools Trybots on Go repo CLs that use generics.

This issue tracks that process (skipping the test, updating the exporter, and unskipping).

CC @griesemer @mdempsky @danscales @ianlancetaylor

[gopherbot]

Change https://golang.org/cl/349954 mentions this issue: go/internal/gcimporter: skip TestIExportData_stdlib on go1.18

[gopherbot]

Change https://golang.org/cl/351029 mentions this issue: go/internal/gcimporter: add support for generics to iexport.go