Limit symbol length in demangle()

Limit symbol length in demangle()

Summary:
GCC's `__cxa_demangle()` uses on-stack data structures for the parser
state which are linear in the number of components of the symbol. For
extremely long symbols, this can cause a stack overflow.

This diff introduces an arbitrary symbol length limit above which we
just return the mangled name.

Reviewed By: philippv

Differential Revision: D3641115

fbshipit-source-id: ec360bb20ca499fd0eaf3a06c5bbcbd1e936d845