compiler: try harder to find source locations for constants

If there is no source location, at least use the file (but not line) for
debug information.

This attributes almost 1kB of string data for ./testdata/stdlib.go when
compiling for WASI.

compiler/compiler.go

@@ -1863,6 +1863,14 @@ func (b *builder) createFunctionCall(instr *ssa.CallCommon) (llvm.Value, error)
 func (b *builder) getValue(expr ssa.Value, pos token.Pos) llvm.Value {
 	switch expr := expr.(type) {
 	case *ssa.Const:
+		if pos == token.NoPos {
+			// If the position isn't known, at least try to find in which file
+			// it is defined.
+			file := b.program.Fset.File(b.fn.Pos())
+			if file != nil {
+				pos = file.Pos(0)
+			}
+		}
 		return b.createConst(expr, pos)
 	case *ssa.Function:
 		if b.getFunctionInfo(expr).exported {