[compiler-rt] Apply modernize-use-nullptr fixes in sanitizers

- Trim spaces.
- Use nullptr in place of 0 for pointer variables.
- Use '!p' in place of 'p == 0' for null pointer checks.
- Add blank lines to separate function definitions.
- Add 'extern "C"' or 'namespace foo' comments after the appropriate
  closing brackets

This is a continuation of work from 409b7b82. The focus here is on the
various sanitizers (not sanitizer_common, as before).

Patch by Eugene Zelenko!

Differential Revision: http://reviews.llvm.org/D13225

llvm-svn: 248966