Improve readability of "undefined reference" message (#82671)

The current message implies a command line flag caused an undefined
reference. This of course is wrong and causes confusion. The message now
more accurately reflects the true state of affairs.

Author: DeanSturtevant1

lld/ELF/Writer.cpp

@@ -2057,8 +2057,9 @@ template <class ELFT> void Writer<ELFT>::finalizeSections() {
           if (sym->dsoDefined)
             continue;
           if (sym->isUndefined() && !sym->isWeak()) {
-            diagnose("undefined reference due to --no-allow-shlib-undefined: " +
-                     toString(*sym) + "\n>>> referenced by " + toString(file));
+            diagnose("undefined reference: " + toString(*sym) +
+                     "\n>>> referenced by " + toString(file) +
+                     " (disallowed by --no-allow-shlib-undefined)");
           } else if (sym->isDefined() && sym->computeBinding() == STB_LOCAL) {
             diagnose("non-exported symbol '" + toString(*sym) + "' in '" +
                      toString(sym->file) + "' is referenced by DSO '" +

lld/test/ELF/allow-shlib-undefined.s

@@ -43,19 +43,19 @@
 # RUN: ld.lld --gc-sections main.o a.so def.so def-hidden.o --fatal-warnings -o /dev/null
 
 # CHECK-NOT:   error:
-# CHECK:       error: undefined reference due to --no-allow-shlib-undefined: x1{{$}}
-# CHECK-NEXT:  >>> referenced by a.so{{$}}
+# CHECK:       error: undefined reference: x1{{$}}
+# CHECK-NEXT:  >>> referenced by a.so (disallowed by --no-allow-shlib-undefined){{$}}
 # CHECK-NOT:   {{.}}
 
 # CHECK2-NOT:  error:
-# CHECK2:      error: undefined reference due to --no-allow-shlib-undefined: x1
-# CHECK2-NEXT: >>> referenced by a.so
-# CHECK2:      error: undefined reference due to --no-allow-shlib-undefined: x1
-# CHECK2-NEXT: >>> referenced by b.so
+# CHECK2:      error: undefined reference: x1
+# CHECK2-NEXT: >>> referenced by a.so (disallowed by --no-allow-shlib-undefined)
+# CHECK2:      error: undefined reference: x1
+# CHECK2-NEXT: >>> referenced by b.so (disallowed by --no-allow-shlib-undefined)
 # CHECK2-NOT:  {{.}}
 
-# WARN:        warning: undefined reference due to --no-allow-shlib-undefined: x1
-# WARN-NEXT:   >>> referenced by a.so
+# WARN:        warning: undefined reference: x1
+# WARN-NEXT:   >>> referenced by a.so (disallowed by --no-allow-shlib-undefined)
 
 # NONEXPORTED-NOT: error:
 # NONEXPORTED:     error: non-exported symbol 'x1' in 'def-hidden.o' is referenced by DSO 'a.so'

lld/test/ELF/unresolved-symbols.s

@@ -36,7 +36,7 @@
 ## --unresolved-symbols overrides a previous --allow-shlib-undefined.
 # RUN: not ld.lld %t1.o %t.so -o /dev/null --allow-shlib-undefined --unresolved-symbols=ignore-in-object-files 2>&1 | FileCheck %s --check-prefix=SHLIB
 
-# SHLIB: error: undefined reference due to --no-allow-shlib-undefined: undef
+# SHLIB: error: undefined reference: undef
 
 ## Ignoring undefines in shared should produce error for symbol from object.
 # RUN: not ld.lld %t2.o -o /dev/null --unresolved-symbols=ignore-in-shared-libs 2>&1 | \

lld/test/ELF/wrap-shlib-undefined.s

@@ -17,7 +17,7 @@
 
 ## --no-allow-shlib-undefined errors because __real_foo is not defined.
 # RUN: not ld.lld %t/main.o %t/bar.so -o /dev/null 2>&1 | FileCheck --check-prefix=ERR %s
-# ERR: error: undefined reference due to --no-allow-shlib-undefined: __real_foo
+# ERR: error: undefined reference: __real_foo
 
 ## --wrap=foo defines __real_foo.
 # RUN: ld.lld %t/main.o %t/bar.so --wrap=foo -o %t2