Demangle: only demangle mangled symbols

Only attempt to demangle symbols which have the itanium C++ prefix of `_Z`.
This ensures that we do not treat any symbol name as a managled named.  We would
previously treat a C function `f` as a mangled name and decode that to `float`
incorrectly.

While it is easy to add tests for this, Mehdi recommended against introducing
tests for the demangler as libc++abi should cover the testing.

llvm-svn: 286795