[ubsan] Improve diagnostics for return value checks (clang)

This patch makes ubsan's nonnull return value diagnostics more precise,
which makes the diagnostics more useful when there are multiple return
statements in a function. Example:

1 |__attribute__((returns_nonnull)) char *foo() {
2 |  if (...) {
3 |    return expr_which_might_evaluate_to_null();
4 |  } else {
5 |    return another_expr_which_might_evaluate_to_null();
6 |  }
7 |} // <- The current diagnostic always points here!

runtime error: Null returned from Line 7, Column 2!
With this patch, the diagnostic would point to either Line 3, Column 5
or Line 5, Column 5.

This is done by emitting source location metadata for each return
statement in a sanitized function. The runtime is passed a pointer to
the appropriate metadata so that it can prepare and deduplicate reports.

Compiler-rt patch (with more tests): https://reviews.llvm.org/D34298

Differential Revision: https://reviews.llvm.org/D34299

llvm-svn: 306163