[NativePDB] Higher fidelity reconstruction of AST from Debug Info.

In order to accurately put a type into the correct location in the AST
we construct from debug info, we need to be able to determine what
DeclContext (namespace, global, nested class, etc) that it goes into.
PDB doesn't contain this mapping.  It does, however, contain the reverse
mapping.  That is, for a given class type T, you can determine all
classes Q1, Q2, ..., Qn that are nested inside of T.  We need to know,
for a given class type Q, what type T is it nested inside of.

This patch builds this map as a pre-processing step when we first
load the PDB by scanning every type.  Initial tests show that while
this can be slow in debug builds of LLDB, it is quite fast in release
builds (less than 2 seconds for a ~1GB PDB, and it only needs to happen
once).

Furthermore, having this pre-processing step in place allows us to
repurpose it for building up other kinds of indexing to it down the
line.  For the time being, this gives us very accurate reconstruction
of the DeclContext hierarchy.

Differential Revision: https://reviews.llvm.org/D54216

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346429 91177308-0d34-0410-b5e6-96231b3b80d8

Author: Zachary Turner

include/llvm/DebugInfo/CodeView/TypeRecord.h

@@ -429,6 +429,10 @@ class TagRecord : public TypeRecord {
     return (Options & ClassOptions::ForwardReference) != ClassOptions::None;
   }
 
+  bool containsNestedClass() const {
+    return (Options & ClassOptions::ContainsNestedClass) != ClassOptions::None;
+  }
+
   bool isScoped() const {
     return (Options & ClassOptions::Scoped) != ClassOptions::None;
   }