[clang-tidy] fix false-positive for cppcoreguidelines-pro-type-member-init...

[clang-tidy] fix false-positive for cppcoreguidelines-pro-type-member-init with in-class initializers

Summary:
This fixes https://llvm.org/bugs/show_bug.cgi?id=30487 where
```
warning: uninitialized record type: 's' [cppcoreguidelines-pro-type-member-init]
```
is emitted on
```
struct MyStruct
{
    int a = 5;
    int b = 7;
};

int main()
{
    MyStruct s;
}
```

Reviewers: alexfh, aaron.ballman

Subscribers: nemanjai, cfe-commits

Differential Revision: https://reviews.llvm.org/D24848

llvm-svn: 282625