[C++17] Reject shadowing of capture by parameter in lambda

Summary:
This change rejects the shadowing of a capture by a parameter in lambdas in C++17.

```
int main() {
  int a;
  auto f = [a](int a) { return a; };
}
```

results in:

```
main.cpp:3:20: error: a lambda parameter cannot shadow an explicitly captured entity
  auto f = [a](int a) { return a; };
                   ^
main.cpp:3:13: note: variable a is explicitly captured here
  auto f = [a](int a) { return a; };
            ^
```

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: lebedev.ri, erik.pilkington, cfe-commits

Differential Revision: https://reviews.llvm.org/D53595

llvm-svn: 345308