oracle: skip cgo processing for queries that don't need SSA/PTA

This makes type-based queries faster and more robust because they
needn't invoke a C compiler, at the cost of worse results in/near .go
files that import "C".  It's particularly important for "referrers"
since a refererrs query on a name from the standard library can load
every package in the workspace.

Fixes issue 10347

Change-Id: I2f65474ce963de5c0897ba67eeb26290dd449cf1
Reviewed-on: https://go-review.googlesource.com/8493
Reviewed-by: David Crawshaw <crawshaw@golang.org>

oracle/oracle.go

@@ -278,6 +278,9 @@ func parseQueryPos(lprog *loader.Program, posFlag string, needExact bool) (*quer
 // allowErrors causes type errors to be silently ignored.
 // (Not suitable if SSA construction follows.)
 func allowErrors(lconf *loader.Config) {
+	ctxt := *lconf.Build // copy
+	ctxt.CgoEnabled = false
+	lconf.Build = &ctxt
 	lconf.AllowErrors = true
 	lconf.TypeChecker.Error = func(err error) {}
 }