[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
diff --git a/llvm/lib/TextAPI/ELF/TBEHandler.cpp b/llvm/lib/TextAPI/ELF/TBEHandler.cpp
index dde9802..b621829 100644
--- a/llvm/lib/TextAPI/ELF/TBEHandler.cpp
+++ b/llvm/lib/TextAPI/ELF/TBEHandler.cpp
@@ -105,6 +105,7 @@
       IO.mapRequired("Size", Symbol.Size);
     }
     IO.mapOptional("Undefined", Symbol.Undefined, false);
+    IO.mapOptional("Weak", Symbol.Weak, false);
     IO.mapOptional("Warning", Symbol.Warning);
   }