[sanitizer] Set IndentPPDirectives: AfterHash in .clang-format

Code patterns like this are common, `#` at the line beginning
(https://google.github.io/styleguide/cppguide.html#Preprocessor_Directives),
one space indentation for if/elif/else directives.
```
#if SANITIZER_LINUX
# if defined(__aarch64__)
# endif
#endif
```

However, currently clang-format wants to reformat the code to
```
#if SANITIZER_LINUX
#if defined(__aarch64__)
#endif
#endif
```

This significantly harms readability in my review.  Use `IndentPPDirectives:
AfterHash` to defeat the diagnostic. clang-format will now suggest:

```
#if SANITIZER_LINUX
#  if defined(__aarch64__)
#  endif
#endif
```

Unfortunately there is no clang-format option using indent with 1 for
just preprocessor directives. However, this is still one step forward
from the current behavior.

Reviewed By: #sanitizers, vitalybuka

Differential Revision: https://reviews.llvm.org/D100238