Merge SymbolBody and Symbol into one class, SymbolBody.

SymbolBody and Symbol were separated classes due to a historical reason.
Symbol used to be a pointer to a SymbolBody, and the relationship
between Symbol and SymbolBody was n:1.

r2681780 changed that. Since that patch, SymbolBody and Symbol are
allocated next to each other to improve memory locality, and they have
1:1 relationship now. So, the separation of Symbol and SymbolBody no
longer makes sense.

This patch merges them into one class. In order to avoid updating too
many places, I chose SymbolBody as a unified name. I'll rename it Symbol
in a follow-up patch.

Differential Revision: https://reviews.llvm.org/D39406

llvm-svn: 317006