This Go frontend patch by Cherry Mui supports the new language
constructs in escape analysis.  Previous patches added new language
constructs in Go 1.17, specifically, unsafe.Add, unsafe.Slice, and
conversion from a slice to a pointer
to an array. This patch handles them in the escape analysis.

At the point of the escape analysis, unsafe.Add and unsafe.Slice are
still builtin calls, so just handle them in data flow.  Conversion
from a slice to a pointer to an array has already been lowered to a
combination of compound expression, conditional expression and slice
info expressions, so handle them in the escape analysis.

Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
to mainline.

Ian