[clang-tidy] Add check to flag use of early `return` or `continue` (readability-guard-clause)

Code like this:
```cpp
void foo() {
// some processing

if (cond) {
// a lot of code
}
}
```
can be transformed to:
```cpp
void foo() {
// some processing

if (!cond)
return

// a lot of code
}
```

And code like this:
```cpp
while (...) {
if (cond) {
// a lot of code
}
}
```
can be transformed to:
```cpp
while (...) { // could be 'for' too
if (!cond)
continue
// a lot of code
}
```