Change `file_symtabs' to std::vector

Change `file_symtabs' to std::vector

This patch changes the `file_symtabs' members in linespec.c structures
from a VEC to a std::vector (or unique_ptr thereof), eliminating a cleanup
in the process.

gdb/ChangeLog:

	* linespec.c (symtab_vector_up): Define.
	(struct linespec) <file_symtabs>: Change type to std::vector *.
	Update all uses.
	(struct collect_info) <file_symtabs>: Likewise.
	(collect_symtabs_from_filename): Return symtab_vector_up.
	Update all callers.
	(decode_objc): Remove cleanup.
	(symtab_collector::symtab_collector): Initialize `m_symtabs'.
	(symtab_collector::release_symtabs): Return symtab_vector_up.
	Update all callers.
	(class symtab_collector) <m_symtabs>: Change type to symtab_vector_up.
	Update all users.
	(collect_symtabs_from_filename, symtabs_from_filename): Return
	symtab_vector_up.  Update all callers.