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

[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

llvm-svn: 327558