Move to the libc++ demangler.

The previous versions of the libc++ demangler crashed on bad input.
However, the new version passes a fuzzer and has a lot of tests. Since
it's more complete than the local demangler, use it instead.

Modified the expected output of an offline test since the new demangler
handles a case that didn't work before.

Verified that the time it takes for the check_for_leak tests did not
change after this.

Bug: 136138882

Test: Ran the unit tests.
Test: Verified the __cxa_demangle function passes the fuzzer when run for
Test: hours. Both the 32 bit and 64 bit version of __cxa_demangle were
Test: fuzzed using external/libcxxabi/fuzz.
Change-Id: I10c06b589d57c36d89dbecba020b1ef2da69634a