[SymbolFile] Rename ParseFunctionBlocks to ParseBlocksRecursive.

This method took a SymbolContext but only actually cared about the
case where the m_function member was set.  Furthermore, it was
intended to be implemented to parse blocks recursively despite not
documenting this in its name.  So we change the name to indicate
that it should be recursive, while also limiting the function
parameter to be a Function&.  This lets the caller know what is
required to use it, as well as letting new implementers know what
kind of inputs they need to be prepared to handle.

llvm-svn: 351131