testsuite: Add dg-lto-error directive support to lto.exp.

The LTO test framework lacked support for testing expected link-time
errors.  Tests could use dg-lto-warning and dg-lto-message for
diagnostics emitted during compilation, but there was no way to verify
that certain code patterns correctly fail at link time under LTO.

This patch adds dg-lto-error directive support, allowing tests to:

1. Mark expected link failures with dg-lto-error directives
2. Pass when the link fails as expected (no executable created or
   linker errors present)
3. Fail when the link succeeds but should have failed
4. Skip execution tests when link failure is expected

The implementation tracks dg-lto-error presence via a global flag and
performs custom link result checking when link failure is expected,
rather than using the standard check_compile reporting.

Signed-off-by: Luis Silva <luiss@synopsys.com>

Author: luismgsilva

gcc/testsuite/lib/lto.exp

@@ -323,6 +323,7 @@ proc lto-link-and-maybe-run { testname objlist dest optall optfile optstr } {
     global tool
     global compile_type
     global board_info
+    global dg_lto_has_error
 
     upvar dg-messages-by-file dg-messages-by-file
 
@@ -370,8 +371,31 @@ proc lto-link-and-maybe-run { testname objlist dest optall optfile optstr } {
     # Prune unimportant visibility warnings before checking output.
     set comp_output [lto_prune_warns $comp_output]
 
-    if ![${tool}_check_compile "$testcase $testname link" $optstr \
-	 $dest $comp_output] then {
+    # Check if link succeeded
+    # If we expect errors, check manually without reporting
+    if { [info exists dg_lto_has_error] && $dg_lto_has_error } {
+	verbose "lto.exp: dg-lto-error present, checking for expected link failure" 2
+	# Check if link failed (executable not created or output has errors)
+	set link_failed [expr {![file_on_host exists $dest] || $comp_output != ""}]
+	if { $link_failed } {
+	    # Link failure is expected due to dg-lto-error - pass the test
+	    verbose "lto.exp: link failed as expected" 2
+	    pass "$testcase $testname link $optstr"
+	} else {
+	    # Link succeeded when it should have failed
+	    fail "$testcase $testname link $optstr (expected failure)"
+	}
+	if { ![string compare "execute" $compile_type] } {
+	    unresolved "$testcase $testname execute $optstr"
+	}
+	return
+    }
+    
+    # Normal link checking (no dg-lto-error)
+    set link_ok [${tool}_check_compile "$testcase $testname link" $optstr \
+	 $dest $comp_output]
+    
+    if { !$link_ok } {
 	if { ![string compare "execute" $compile_type] } {
 	    unresolved "$testcase $testname execute $optstr"
 	}
@@ -403,17 +427,25 @@ proc lto-can-handle-directive { op } {
 
     # dg-warning and dg-message append to dg-messages.
     upvar dg-messages dg-messages
+    # Track if we have dg-lto-error to expect link failure
+    global dg_lto_has_error
 
     # A list of directives to recognize, and a list of directives
     # to remap them to.
     # For example, "dg-lto-warning" is implemented by calling "dg-warning".
-    set directives { dg-lto-warning dg-lto-message dg-lto-note }
-    set remapped_directives { dg-warning dg-message dg-note }
+    set directives { dg-lto-warning dg-lto-message dg-lto-note dg-lto-error }
+    set remapped_directives { dg-warning dg-message dg-note dg-error }
 
     set idx [lsearch -exact $directives $cmd]
     if { $idx != -1 } {
 	verbose "remapping from: $op" 4
 
+	# Mark that we have a dg-lto-error directive
+	if { $cmd == "dg-lto-error" } {
+	    set dg_lto_has_error 1
+	    verbose "lto.exp: dg-lto-error detected, expecting link failure" 2
+	}
+
 	set remapped_cmd [lindex $remapped_directives $idx]
 	set op [lreplace $op 0 0 $remapped_cmd]
 
@@ -629,6 +661,7 @@ proc lto-execute-1 { src1 sid } {
     global LTO_OPTIONS
     global dg-final-code
     global testname_with_flags
+    global dg_lto_has_error
 
     # Get extra flags for this test from the primary source file, and
     # process other dg-* options that this suite supports.  Warn about
@@ -637,6 +670,7 @@ proc lto-execute-1 { src1 sid } {
     set compile_type "run"
     set dg-do-what [list ${dg-do-what-default} "" P]
     array set dg-messages-by-file [list]
+    set dg_lto_has_error 0
     set extra_flags(0) [lto-get-options-main $src1]
     set compile_xfail(0) ""