[TextAPI][elfabi] Fix YAML support for weak symbols

Weak symbols are supposed to be supported in the ELF TextAPI
implementation, but the YAML handler didn't read or write the `Weak`
member of ELFSymbol. This change adds the YAML mapping and updates tests
to ensure correct behavior.

Differential Revision: https://reviews.llvm.org/D56020

llvm-svn: 349950