Remove injection of -Werror=implicit-function-declaration (emscripten-core#23465)

I believe this change in the defaults was probably don't back in the day
when mismatched function pointers were not otherwise will reported and
could lead to hard-to-debug crashes. These days the linker will five a
clear error about function signature mistmatches between object files so
I think the deviation is no longer worth it.

This change only effect C and not C++.

Note: Adding this extra warning actually has some cost because it
requires us to understand if the compiler is running C or C++ mode. This
is more complicated than is can seem because its possible to specify
more than one language mode on the command line. e.g:

clang -x c++ myfile.cpp -x c myfile.c

We don't currently have great support for this method, and its obviously
not common.

Author: sbc100

emcc.py

@@ -424,7 +424,7 @@ def get_clang_flags(user_args):
 cflags = None
 
 
-def get_cflags(user_args, is_cxx):
+def get_cflags(user_args):
   global cflags
   if cflags:
     return cflags
@@ -448,17 +448,6 @@ def get_cflags(user_args, is_cxx):
     # in strict mode. Code should use the define __EMSCRIPTEN__ instead.
     cflags.append('-DEMSCRIPTEN')
 
-  # Changes to default clang behavior
-
-  # Implicit functions can cause horribly confusing function pointer type errors, see #2175
-  # If your codebase really needs them - very unrecommended! - you can disable the error with
-  #   -Wno-error=implicit-function-declaration
-  # or disable even a warning about it with
-  #   -Wno-implicit-function-declaration
-  # This is already an error in C++ so we don't need to inject extra flags.
-  if not is_cxx:
-    cflags += ['-Werror=implicit-function-declaration']
-
   ports.add_cflags(cflags, settings)
 
   def array_contains_any_of(hay, needles):
@@ -984,10 +973,9 @@ def get_language_mode(args):
     return ''
 
   language_mode = get_language_mode(newargs)
-  use_cxx = 'c++' in language_mode or shared.run_via_emxx
 
   def get_clang_command():
-    return compiler + get_cflags(state.orig_args, use_cxx)
+    return compiler + get_cflags(state.orig_args)
 
   def get_clang_command_preprocessed():
     return compiler + get_clang_flags(state.orig_args)

test/other/test_implicit_func.c

@@ -5368,18 +5368,6 @@ def test_global_inits(self):
       self.assertContained('argc: 1\n16\n17\n10\n', self.run_js('a.out.js'))
       self.assertContainedIf('globalCtors', src, has_global)
 
-  def test_implicit_func(self):
-    # EMCC makes -Wimplicit-function-declaration an error by default in all modes. Upstream LLVM
-    # emits a warning in gnu89 mode, but otherwise emcc's behavior is identical to upstream.
-    IMPLICIT_C89 = "error: implicit declaration of function 'strnlen'"
-    # Also check for -Wincompatible-function-pointer-types (it became an error in LLVM 16)
-    INCOMPATIBLE = ': incompatible function pointer types'
-
-    stderr = self.expect_fail(
-        [EMCC, path_from_root('test/other/test_implicit_func.c'), '-c', '-o', 'implicit_func.o', '-std=gnu89'])
-    self.assertContained(IMPLICIT_C89, stderr)
-    self.assertContained(INCOMPATIBLE, stderr)
-
   @requires_native_clang
   @crossplatform
   def test_bad_triple(self):