[clang] Reland "[clang] Add nuw attribute to GEPs (#105496)" (PR #107257)

[Arseny Kapoulkine via cfe-commits]

zeux wrote:

Here's a slightly smaller reproducer that just reads one element from the array. Similarly, this crashes because the compiler generates a load from an absolute `-1` address. The crash is after the first `printf` call in this case during the computation of the argument to the second `printf` call. (the code is fairly non-sensical in this revision, but maybe it helps to analyze this!).

```c++
#include <stdio.h>
#include <stdint.h>
#include <string.h>

class xml_buffered_writer
{
public:
	xml_buffered_writer(): bufsize(0)
	{
	}

	__attribute__((noinline))
	void write_string(const char* data)
	{
		// write the part of the string that fits in the buffer
		size_t offset = bufsize;

		while (*data && offset < bufcapacity)
			buffer[offset++] = *data++;

		// write the rest
		if (offset < bufcapacity)
		{
			bufsize = offset;
		}
		else
		{
			// backtrack a bit if we have split the codepoint
			size_t length = offset - bufsize;
			const char* data_back = data - length;
			printf("length %d\n", int(length));
			printf("last char %c\n", data_back[length-1]);

			bufsize = offset;
		}
	}

	enum
	{
		bufcapacity = 16
	};

	char buffer[bufcapacity];
	size_t bufsize;
};

int main()
{
	xml_buffered_writer writer;
	writer.write_string("abcdefghijklmnopqrstuvwxyz");
	printf("\n");
}
```


https://github.com/llvm/llvm-project/pull/107257