Disable -Wmissing-prototypes for internal linkage functions that aren't explicitly marked "static"

Some functions can end up non-externally visible despite not being
declared "static" or in an unnamed namespace in C++ - such as by having
parameters that are of non-external types.

Such functions aren't mistakenly intended to be defining some function
that needs a declaration. They could be maybe more legible (except for
the operator new example) with an explicit static, but that's a
stylistic thing outside what should be addressed by a warning.

This reapplies 275c56226d - once we figure
out what to do about the change in behavior for -Wnon-c-typedef-for-linkage
(this reverts the revert commit 85ee1d3ca1)

Differential Revision: https://reviews.llvm.org/D121328

clang/lib/Sema/SemaDecl.cpp

@@ -14233,6 +14233,11 @@ ShouldWarnAboutMissingPrototype(const FunctionDecl *FD,
   if (FD->isDeleted())
     return false;
 
+  // Don't warn on implicitly local functions (such as having local-typed
+  // parameters).
+  if (!FD->isExternallyVisible())
+    return false;
+
   for (const FunctionDecl *Prev = FD->getPreviousDecl();
        Prev; Prev = Prev->getPreviousDecl()) {
     // Ignore any declarations that occur in function or method

clang/test/SemaCXX/anonymous-struct.cpp

@@ -183,3 +183,9 @@ namespace BuiltinName {
     void memcpy(); // expected-note {{due to this member}}
   } A; // expected-note {{given name 'A' for linkage purposes by this typedef}}
 }
+namespace inline_defined_static_member {
+typedef struct { // expected-warning {{anonymous non-C-compatible type}}
+  static void f() { // expected-note {{due to this member}}
+  }
+} A; // expected-note {{given name 'A' for linkage purposes by this typedef}}
+}

clang/test/SemaCXX/warn-missing-prototypes.cpp

@@ -44,3 +44,16 @@ void j() = delete;
 extern void k() {} // expected-warning {{no previous prototype for function 'k'}}
 // expected-note@-1{{declare 'static' if the function is not intended to be used outside of this translation unit}}
 // CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-2]]:{{.*}}-[[@LINE-2]]:{{.*}}}:"{{.*}}"
+
+namespace {
+struct anon { };
+}
+
+// No warning because this has internal linkage despite not being declared
+// explicitly 'static', owing to the internal linkage parameter.
+void l(anon) {
+}
+
+void *operator new(decltype(sizeof(3)) size, const anon &) throw() {
+  return nullptr;
+}