[ASTImporter] Fix redecl chain of classes and class templates

Summary:
The crux of the issue that is being fixed is that lookup could not find
previous decls of a friend class. The solution involves making the
friend declarations visible in their decl context (i.e. adding them to
the lookup table).
Also, we simplify `VisitRecordDecl` greatly.

This fix involves two other repairs (without these the unittests fail):
(1) We could not handle the addition of injected class types properly
when a redecl chain was involved, now this is fixed.
(2) DeclContext::removeDecl failed if the lookup table in Vector form
did not contain the to be removed element. This caused troubles in
ASTImporter::ImportDeclContext. This is also fixed.

Reviewers: a_sidorin, balazske, a.sidorin

Subscribers: rnkovacs, dkrupp, Szelethus, cfe-commits

Differential Revision: https://reviews.llvm.org/D53655

llvm-svn: 349349
diff --git a/clang/docs/LibASTMatchersReference.html b/clang/docs/LibASTMatchersReference.html
index 28017ca..27cb00b 100644
--- a/clang/docs/LibASTMatchersReference.html
+++ b/clang/docs/LibASTMatchersReference.html
@@ -300,6 +300,16 @@
 </pre></td></tr>
 
 
+<tr><td>Matcher&lt;<a href="https://clang.llvm.org/doxygen/classclang_1_1Decl.html">Decl</a>&gt;</td><td class="name" onclick="toggle('indirectFieldDecl0')"><a name="indirectFieldDecl0Anchor">indirectFieldDecl</a></td><td>Matcher&lt;<a href="https://clang.llvm.org/doxygen/classclang_1_1IndirectFieldDecl.html">IndirectFieldDecl</a>&gt;...</td></tr>
+<tr><td colspan="4" class="doc" id="indirectFieldDecl0"><pre>Matches indirect field declarations.
+
+Given
+  struct X { struct { int a; }; };
+indirectFieldDecl()
+  matches 'a'.
+</pre></td></tr>
+
+
 <tr><td>Matcher&lt;<a href="https://clang.llvm.org/doxygen/classclang_1_1Decl.html">Decl</a>&gt;</td><td class="name" onclick="toggle('labelDecl0')"><a name="labelDecl0Anchor">labelDecl</a></td><td>Matcher&lt;<a href="https://clang.llvm.org/doxygen/classclang_1_1LabelDecl.html">LabelDecl</a>&gt;...</td></tr>
 <tr><td colspan="4" class="doc" id="labelDecl0"><pre>Matches a declaration of label.