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) {
+}

compiler/testdata/pragma.ll

@@ -85,6 +85,14 @@ entry:
 
 declare void @main.undefinedFunctionNotInSection(ptr) #1
 
+declare void @main.doesNotEscapeParam(ptr nocapture dereferenceable_or_null(4), ptr nocapture, i32, i32, ptr nocapture dereferenceable_or_null(32), ptr nocapture, ptr) #1
+
+; Function Attrs: nounwind
+define hidden void @main.stillEscapes(ptr dereferenceable_or_null(4) %a, ptr %b.data, i32 %b.len, i32 %b.cap, ptr dereferenceable_or_null(32) %c, ptr %d, ptr %context) unnamed_addr #2 {
+entry:
+  ret void
+}
+
 attributes #0 = { allockind("alloc,zeroed") allocsize(0) "alloc-family"="runtime.alloc" "target-features"="+bulk-memory,+mutable-globals,+nontrapping-fptoint,+sign-ext" }
 attributes #1 = { "target-features"="+bulk-memory,+mutable-globals,+nontrapping-fptoint,+sign-ext" }
 attributes #2 = { nounwind "target-features"="+bulk-memory,+mutable-globals,+nontrapping-fptoint,+sign-ext" }