[Parser] (C++) Make -Wextra-semi slightly more useful

Summary:
Let's suppose the `-Weverything` is passed.

Given code like
```
void F() {}
;
```
If the code is compiled with `-std=c++03`, it would diagnose that extra sema:
```
<source>:2:1: warning: extra ';' outside of a function is a C++11 extension [-Wc++11-extra-semi]
;
^~
```
If the code is compiled with `-std=c++11`, it also would diagnose that extra sema:
```
<source>:2:1: warning: extra ';' outside of a function is incompatible with C++98 [-Wc++98-compat-pedantic]
;
^~
```

But, let's suppose the C++11 or higher is used, and the used does not care
about `-Wc++98-compat-pedantic`, so he disables that diagnostic.
And that silences the complaint about extra `;` too.
And there is no way to re-enable that particular diagnostic, passing `-Wextra-semi` does nothing...

Now, there is also a related `no newline at end of file` diagnostic, which is also emitted by `-Wc++98-compat-pedantic`
```
<source>:2:2: warning: C++98 requires newline at end of file [-Wc++98-compat-pedantic]
;
 ^
```
But unlike the previous case, if `-Wno-c++98-compat-pedantic` is passed, that diagnostic stays displayed:
```
<source>:2:2: warning: no newline at end of file [-Wnewline-eof]
;
 ^
```

This diff refactors the code so `-Wc++98-compat-extra-semi` can be re-enabled, after the `-Wc++98-compat-pedantic` was disabled.
This seems ugly, but there does not seem to be any saner way.

Testing: `$ ninja check-clang`

Reviewers: rsmith, rtrieu, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: jordan_rose, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D43162

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@327558 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Basic/DiagnosticGroups.td

@@ -151,8 +151,10 @@ def Exceptions : DiagGroup<"exceptions">;
 def GNUEmptyInitializer : DiagGroup<"gnu-empty-initializer">;
 def GNUEmptyStruct : DiagGroup<"gnu-empty-struct">;
 def ExtraTokens : DiagGroup<"extra-tokens">;
+def CXX98CompatExtraSemi : DiagGroup<"c++98-compat-extra-semi">;
 def CXX11ExtraSemi : DiagGroup<"c++11-extra-semi">;
-def ExtraSemi : DiagGroup<"extra-semi", [CXX11ExtraSemi]>;
+def ExtraSemi : DiagGroup<"extra-semi", [CXX98CompatExtraSemi,
+                                         CXX11ExtraSemi]>;
 
 def GNUFlexibleArrayInitializer : DiagGroup<"gnu-flexible-array-initializer">;
 def GNUFlexibleArrayUnionMember : DiagGroup<"gnu-flexible-array-union-member">;
@@ -196,6 +198,7 @@ def CXX98Compat : DiagGroup<"c++98-compat",
 def CXX98CompatPedantic : DiagGroup<"c++98-compat-pedantic",
                                     [CXX98Compat,
                                      CXX98CompatBindToTemporaryCopy,
+                                     CXX98CompatExtraSemi,
                                      CXXPre14CompatPedantic,
                                      CXXPre17CompatPedantic,
                                      CXXPre2aCompatPedantic]>;

include/clang/Basic/DiagnosticParseKinds.td

@@ -39,7 +39,7 @@ def ext_empty_translation_unit : Extension<
   InGroup<DiagGroup<"empty-translation-unit">>;
 def warn_cxx98_compat_top_level_semi : Warning<
   "extra ';' outside of a function is incompatible with C++98">,
-  InGroup<CXX98CompatPedantic>, DefaultIgnore;
+  InGroup<CXX98CompatExtraSemi>, DefaultIgnore;
 def ext_extra_semi : Extension<
   "extra ';' %select{"
   "outside of a function|"

test/Parser/cxx-extra-semi.cpp

@@ -38,4 +38,7 @@ union B {
 #if __cplusplus < 201103L
 // expected-warning@-3{{extra ';' outside of a function is a C++11 extension}}
 // expected-warning@-3{{extra ';' outside of a function is a C++11 extension}}
+#elif !defined(PEDANTIC)
+// expected-warning@-6{{extra ';' outside of a function is incompatible with C++98}}
+// expected-warning@-6{{extra ';' outside of a function is incompatible with C++98}}
 #endif

test/SemaCXX/extra-semi.cpp

@@ -0,0 +1,29 @@
+// RUN: %clang_cc1 -verify -std=c++98 -Wextra-semi %s
+// RUN: %clang_cc1 -verify -std=c++03 -Wextra-semi %s
+// RUN: %clang_cc1 -verify -std=c++11 -Wextra-semi %s
+// RUN: %clang_cc1 -verify -std=c++17 -Wextra-semi %s
+// RUN: %clang_cc1 -verify -std=c++2a -Wextra-semi %s
+// RUN: %clang_cc1 -verify -Weverything -Wno-c++98-compat %s
+// RUN: %clang_cc1 -verify -Weverything -Wno-c++98-compat-pedantic -Wc++98-compat-extra-semi %s
+
+// Last RUN line checks that c++98-compat-extra-semi can still be re-enabled.
+
+void F();
+
+void F(){}
+;
+#if __cplusplus < 201103L
+// expected-warning@-2{{extra ';' outside of a function is a C++11 extension}}
+#else
+// expected-warning@-4{{extra ';' outside of a function is incompatible with C++98}}
+#endif
+
+namespace ns {
+class C {
+  void F() const;
+};
+}
+; // expected-warning {{extra ';' outside of a function is}}
+
+void ns::C::F() const {}
+; // expected-warning {{extra ';' outside of a function is}}