Deprecate bitcode linking with llvm-link (#15724)

Also, restore some minimal testing of this feature which had been
completely removed.

I don't know of any remaining users of this feature and it adds a fair
amount of complexity to the linking logic.

See #13492

Author: sbc100

tests/test_other.py

@@ -1114,10 +1114,10 @@ def test_link_group_bitcode(self):
 }
 ''')
 
-    self.run_process([EMCC, '-o', '1.o', '-c', '1.c'])
-    self.run_process([EMCC, '-o', '2.o', '-c', '2.c'])
+    self.run_process([EMCC, '-flto', '-o', '1.o', '-c', '1.c'])
+    self.run_process([EMCC, '-flto', '-o', '2.o', '-c', '2.c'])
     self.run_process([EMAR, 'crs', '2.a', '2.o'])
-    self.run_process([EMCC, '-r', '-o', 'out.o', '-Wl,--start-group', '2.a', '1.o', '-Wl,--end-group'])
+    self.run_process([EMCC, '-r', '-flto', '-o', 'out.o', '-Wl,--start-group', '2.a', '1.o', '-Wl,--end-group'])
     self.run_process([EMCC, 'out.o'])
     self.assertContained('Hello', self.run_js('a.out.js'))
 
@@ -3848,9 +3848,10 @@ def test_bc_to_bc(self):
     # e.g. they assume our 'executable' extension is bc, and compile an .o to a .bc
     # (the user would then need to build bc to js of course, but we need to actually
     # emit the bc)
-    self.run_process([EMCC, '-c', test_file('hello_world.c')])
+    self.run_process([EMCC, '-flto', '-c', test_file('hello_world.c')])
     self.assertExists('hello_world.o')
-    self.run_process([EMCC, '-r', 'hello_world.o', '-o', 'hello_world.bc'])
+    err = self.run_process([EMCC, '-flto', '-r', 'hello_world.o', '-o', 'hello_world.bc'], stderr=PIPE).stderr
+    self.assertContained('emcc: warning: bitcode linking with llvm-link is deprecated', err)
     self.assertExists('hello_world.o')
     self.assertExists('hello_world.bc')
 

tools/building.py

@@ -398,6 +398,7 @@ def link_lld(args, target, external_symbols=None):
 
 
 def link_bitcode(args, target, force_archive_contents=False):
+  diagnostics.warning('deprecated', 'bitcode linking with llvm-link is deprecated.  Please use emar archives instead.  See https://github.com/emscripten-core/emscripten/issues/13492')
   # "Full-featured" linking: looks into archives (duplicates lld functionality)
   input_files = [a for a in args if not a.startswith('-')]
   files_to_link = []