[libc++] Remove workaround for PR28391 (ODR violations with ASAN)

[ldionne]

Summary

This is not an issue anymore since we don't build the libc++ dylib with
C++14 anymore (see https://llvm.org/PR28391) for details.

libcxx/utils/libcxx/test/config.py

def configure_sanitizer(self):
                 self.config.available_features.add('ubsan')
 
             # Setup the sanitizer compile flags
             self.cxx.flags += ['-g', '-fno-omit-frame-pointer']
             if san == 'Address' or san == 'Address;Undefined' or san == 'Undefined;Address':
                 self.cxx.flags += ['-fsanitize=address']
                 if llvm_symbolizer is not None:
                     self.exec_env['ASAN_SYMBOLIZER_PATH'] = llvm_symbolizer
-                # FIXME: Turn ODR violation back on after PR28391 is resolved
-                # https://llvm.org/PR28391
-                self.exec_env['ASAN_OPTIONS'] = 'detect_odr_violation=0'
                 self.config.available_features.add('asan')
                 self.config.available_features.add('sanitizer-new-delete')
                 self.cxx.compile_flags += ['-O1']
                 if san == 'Address;Undefined' or san == 'Undefined;Address':
                     add_ubsan()
             elif san == 'Memory' or san == 'MemoryWithOrigins':
                 self.cxx.flags += ['-fsanitize=memory']
                 if san == 'MemoryWithOrigins':