[LTO][Legacy] Add new API to check presence of ctor/dtor functions.

On AIX, the linker needs to check whether a given lto_module_t contains
any constructor/destructor functions, in order to implement the behavior
of the -bcdtors:all flag. See
https://www.ibm.com/docs/en/aix/7.2?topic=l-ld-command for the flag's
documentation.
In llvm IR, constructor (destructor) functions are added to a special
global array @llvm.global_ctors (@llvm.global_dtors).
However, because these two symbols are artificial, they are not visited
during the symbol traversal (using the
lto_module_get_[num_symbols|symbol_name|symbol_attribute] API).

This patch adds a new function to the libLTO interface that checks the
presence of one or both of these two symbols.

Reviewed By: steven_wu

Differential Revision: https://reviews.llvm.org/D106887