DebugInfo: Drop fake DW_TAG_expression

Break MDExpression off of DebugNode (inherit directly from `MDNode`) and
drop the fake `DW_TAG_expression` tag in the process.

AFAICT, there's no real functionality change here.  The tag was
originally used by `DIDescriptor::isExpression()` to discriminate
between `MDNode`s, but in the new hierarchy we don't need that.

Fixes PR22780.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232550 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed