[clang][WebAssembly] Loosen restriction on `main` symbol mangling

[sbc100]

Summary

Remove the hasPrototype() restriction so that old style K&R
declarations of main work too.

For example the following has two params but no prototype.

int main(argc, argv)
    int argc;
    char *argv[];
{
  return 0;
}

Also, use getNumParams() over param_size() which seems to be a more
direct way to get at the same information.

[sbc100]

add tests

clang/lib/AST/Mangle.cpp

static CCMangling getCallingConvMangling(const ASTContext &Context,
                                          const NamedDecl *ND) {
   const TargetInfo &TI = Context.getTargetInfo();
   const llvm::Triple &Triple = TI.getTriple();
 
   // On wasm, the argc/argv form of "main" is renamed so that the startup code
   // can call it with the correct function signature.
   if (Triple.isWasm())
     if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(ND))
-      if (FD->isMain() && FD->hasPrototype() && FD->param_size() == 2)
+      if (FD->isMain() && FD->getNumParams() == 2)
         return CCM_WasmMainArgcArgv;
 
   if (!Triple.isOSWindows() || !Triple.isX86())
     return CCM_Other;
 
   if (Context.getLangOpts().CPlusPlus && !isExternC(ND) &&
       TI.getCXXABI() == TargetCXXABI::Microsoft)
     return CCM_Other;

clang/test/CodeGen/mangle-wasm-main-noproto.c

+// RUN: %clang_cc1 -emit-llvm %s -o - -triple=wasm32-unknown-unknown -Wno-deprecated-non-prototype | FileCheck %s
+
+int main(argc, argv)
+  int argc;
+  char* argv[];
+{
+  return 0;
+}
+// CHECK-NOT: __main_void
+// CHECK: define i32 @__main_argc_argv(i32 noundef %argc, ptr noundef %argv) #0 { 

clang/test/CodeGen/mangle-wasm-main-void.c

+// RUN: %clang_cc1 -emit-llvm %s -o - -triple=wasm32-unknown-unknown | FileCheck %s
+
+int main(void) {
+  return 0;
+}
+// CHECK: @__main_void = hidden alias i32 (), ptr @main
+// CHECK: define i32 @main() #0 {

clang/test/CodeGen/mangle-wasm-main.c

+// RUN: %clang_cc1 -emit-llvm %s -o - -triple=wasm32-unknown-unknown | FileCheck %s
+
+int main(int argc, char* argv[]) {
+  return 0;
+}
+// CHECK-NOT: __main_void
+// CHECK: define i32 @__main_argc_argv(i32 noundef %argc, ptr noundef %argv) #0 {