[LiveDebugValues] Extend var ranges through artificial blocks

ASan often introduces basic blocks consisting exclusively of
instructions without debug locations, or with line 0 debug locations.

LiveDebugValues needs to extend variable ranges through these artificial
blocks. Otherwise, a lot of variables disappear -- even at -O0.

Typically, LiveDebugValues does not extend a variable's range into a
block unless the block is essentially "part of" the variable's scope
(for a precise definition, see LexicalScopes::dominates). This patch
relaxes the lexical dominance check for artificial blocks.

This makes the following Swift program debuggable at -O0:
```
  1| var x = 100
  2| print("x = \(x)")
```

rdar://39127144

Differential Revision: https://reviews.llvm.org/D52921

llvm-svn: 343890