[clang][Parse] Diagnose useless null statements / empty init-statements

Summary:
clang has `-Wextra-semi` (D43162), which is not dictated by the currently selected standard.
While that is great, there is at least one more source of need-less semis - 'null statements'.
Sometimes, they are needed:
```
for(int x = 0; continueToDoWork(x); x++)
  ; // Ugly code, but the semi is needed here.
```

But sometimes they are just there for no reason:
```
switch(X) {
case 0:
  return -2345;
case 5:
  return 0;
default:
  return 42;
}; // <- oops

;;;;;;;;;;; <- OOOOPS, still not diagnosed. Clearly this is junk.
```

Additionally:
```
if(; // <- empty init-statement
   true)
  ;

switch (; // empty init-statement
        x) {
  ...
}

for (; // <- empty init-statement
     int y : S())
  ;
}

As usual, things may or may not go sideways in the presence of macros.
While evaluating this diag on my codebase of interest, it was unsurprisingly
discovered that Google Test macros are *very* prone to this.
And it seems many issues are deep within the GTest itself, not
in the snippets passed from the codebase that uses GTest.

So after some thought, i decided not do issue a diagnostic if the semi
is within *any* macro, be it either from the normal header, or system header.

Fixes [[ https://bugs.llvm.org/show_bug.cgi?id=39111 | PR39111 ]]

Reviewers: rsmith, aaron.ballman, efriedma

Reviewed By: aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D52695

llvm-svn: 347339

0 files changed, 0 insertions, 0 deletions