[sanitizer] Fix a crash when demangling Swift symbols

To invoke the Swift demangler, we use dlsym to locate swift_demangle. However, dlsym malloc's storage and stores it in thread-local storage. Since allocations from the symbolizer are done with the system allocator (at least in TSan, interceptors are skipped when inside the symbolizer), we will crash when we try to deallocate later using the sanitizer allocator again.

To fix this, let's just not call dlsym from the demangler, and call it during initialization.

Differential Revision: http://reviews.llvm.org/D19974

llvm-svn: 268716