[clangd] Add dlog()s for SelectionTree, enabling -debug-only=SelectionTree.cpp

Summary:
SelectionTree is a RecursiveASTVisitor which processes getSourceRange() for
every node. This is a lot of surface area with the AST, as getSourceRange()
is specialized for *many* node types.
And the resulting SelectionTree depends on the source ranges of many
visited nodes, and the order of traversal.

Put together, this means we really need a traversal log to debug when we
get an unexpected SelectionTree. I've built this ad-hoc a few times, now
it's time to check it in.

Example output:
```
D[14:07:44.184] Computing selection for </usr/local/google/home/sammccall/test.cc:1:7, col:8>
D[14:07:44.184]  push: VarDecl const auto x = 42
D[14:07:44.184]   claimRange: </usr/local/google/home/sammccall/test.cc:1:12, col:13>
D[14:07:44.184]   push: NestedNameSpecifierLoc (empty NestedNameSpecifierLoc)
D[14:07:44.184]   pop: NestedNameSpecifierLoc (empty NestedNameSpecifierLoc)
D[14:07:44.184]   push: QualifiedTypeLoc const auto
D[14:07:44.184]   pop: QualifiedTypeLoc const auto
D[14:07:44.184]    claimRange: </usr/local/google/home/sammccall/test.cc:1:7, col:11>
D[14:07:44.184]    hit selection: </usr/local/google/home/sammccall/test.cc:1:7, col:8>
D[14:07:44.184]   skip: IntegerLiteral 42
D[14:07:44.184]    skipped range = </usr/local/google/home/sammccall/test.cc:1:16>
D[14:07:44.184]  pop: VarDecl const auto x = 42
D[14:07:44.184]   claimRange: </usr/local/google/home/sammccall/test.cc:1:1, col:18>
D[14:07:44.184]  skip: VarDecl int y = 43
D[14:07:44.184]   skipped range = </usr/local/google/home/sammccall/test.cc:2:1, col:9>
D[14:07:44.184] Built selection tree
TranslationUnitDecl
  VarDecl const auto x = 42
     .QualifiedTypeLoc const auto

```

Reviewers: hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65073

llvm-svn: 366698