[clang-tidy] New checker for not null-terminated result caused by strlen(), size() or equal length

Summary:
New checker called bugprone-not-null-terminated-result. This checker finds
function calls where it is possible to cause a not null-terminated result.
Usually the proper length of a string is `strlen(src) + 1` or equal length
of this expression, because the null terminator needs an extra space.
Without the null terminator it can result in undefined behaviour when the
string is read.

The following and their respective `wchar_t` based functions are checked:

`memcpy`, `memcpy_s`, `memchr`, `memmove`, `memmove_s`, `strerror_s`,
`strncmp`, `strxfrm`

The following is a real-world example where the programmer forgot to
increase the passed third argument, which is `size_t length`.
That is why the length of the allocated memory is not enough to hold the
null terminator.

```
    static char *stringCpy(const std::string &str) {
      char *result = reinterpret_cast<char *>(malloc(str.size()));
      memcpy(result, str.data(), str.size());
      return result;
    }
```

In addition to issuing warnings, fix-it rewrites all the necessary code.
It also tries to adjust the capacity of the destination array:

```
    static char *stringCpy(const std::string &str) {
      char *result = reinterpret_cast<char *>(malloc(str.size() + 1));
      strcpy(result, str.data());
      return result;
    }
```

Note: It cannot guarantee to rewrite every of the path-sensitive memory
allocations.

Reviewed By: JonasToth, aaron.ballman, whisperity, alexfh

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D45050

llvm-svn: 374707

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