compiler: add //go:noescape pragma

This only works on declarations, not definitions. This is intentional:
it follows the upstream Go implemetation.

However, we might want to loosen this requirement at some point: TinyGo
sometimes stores pointers in memory mapped I/O knowing they won't
actually escape, but the compiler doesn't know about this.

compiler/testdata/pragma.go

@@ -106,3 +106,12 @@ var undefinedGlobalNotInSection uint32
 //go:align 1024
 //go:section .global_section
 var multipleGlobalPragmas uint32
+
+//go:noescape
+func doesNotEscapeParam(a *int, b []int, c chan int, d *[0]byte)
+
+// The //go:noescape pragma only works on declarations, not definitions.
+//
+//go:noescape
+func stillEscapes(a *int, b []int, c chan int, d *[0]byte) {
+}