Fix incorrect form test in SymbolFileDWARF

Summary:
We were checking whether an attribute is in block form by getting the block data pointer, which
was not correct as the pointer be null even if the attribute is in block form. Other places in
the file already use the correct test.

To make this work, I've needed to add DW_FORM_exprlock to the list of "block" forms, which seems
correct as that is how we are parsing it.

Reviewers: clayborg

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D22756

llvm-svn: 276735