Revert "[SLC] Preserve attrs for strncpy(x, "", y) -> memset(align 1 x, '\0', y)"

Revert "[SLC] Preserve attrs for strncpy(x, "", y) -> memset(align 1 x, '\0', y)"

Summary:
This reverts commit r372101.

Causes ASAN build bot failures:

http://lab.llvm.org:8011/builders/sanitizer-ppc64be-linux/builds/14176
From http://lab.llvm.org:8011/builders/sanitizer-ppc64be-linux/builds/14176/steps/64-bit%20check-asan/logs/stdio:

```
[ RUN      ] AddressSanitizer.StrNCatOOBTest
/home/buildbots/ppc64be-sanitizer/sanitizer-ppc64be/build/llvm-project/compiler-rt/lib/asan/tests/asan_str_test.cpp:462: Failure
Death test: strncat(to - 1, from, 0)
    Result: failed to die.
```

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67658

llvm-svn: 372125