[sanitizer] Fix a crash when demangling Swift symbols, take 3

The previous patch (r269291) was reverted (commented out) because the patch caused leaks that
were detected by LSan and they broke some lit tests.  The actual reason was that dlsym allocates
an error string buffer in TLS, and some LSan lit tests are intentionally not scanning TLS for
root pointers.  This patch simply makes LSan ignore the allocation from dlsym, because it's
not interesting anyway.

llvm-svn: 269917