Add some basic extra support for C++ unity building

[makslevental]

See https://lists.llvm.org/pipermail/cfe-dev/2018-April/057579.html

Based on https://github.com/jensl/llvm-project-20170507/tree/wip/jumbo-support/v1

(Unity building here called "jumbo" because it is called that in the context of the Chromium project.)

This adds an example plugin named JumboSupport that adds a pragma named jumbo, which signals that the main source file is a jumbo compilation unit that will include other source files. After each include into the main source file, the contents of all anonymous namespace declarations in the included source file are hidden, so that the anonymous namespace remains local to source files. It also undefines all macros defined in the included source file.

Some (intentionally) minimal changes to clang is implemented to make it possible to implement this functionality in the plugin.

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff HEAD~1 HEAD --extensions cpp,h -- clang/examples/JumboSupport/JumboSupport.cpp clang/include/clang/AST/Decl.h clang/lib/AST/ItaniumMangle.cpp clang/lib/Sema/SemaLookup.cpp

View the diff from clang-format here.

diff --git a/clang/lib/AST/ItaniumMangle.cpp b/clang/lib/AST/ItaniumMangle.cpp
index 17552c23d..2256d54f5 100644
--- a/clang/lib/AST/ItaniumMangle.cpp
+++ b/clang/lib/AST/ItaniumMangle.cpp
@@ -24,12 +24,12 @@
 #include "clang/AST/Expr.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExprConcepts.h"
-#include "clang/Basic/SourceManager.h"
 #include "clang/AST/ExprObjC.h"
 #include "clang/AST/Mangle.h"
 #include "clang/AST/TypeLoc.h"
 #include "clang/Basic/ABI.h"
 #include "clang/Basic/Module.h"
+#include "clang/Basic/SourceManager.h"
 #include "clang/Basic/TargetInfo.h"
 #include "clang/Basic/Thunk.h"
 #include "llvm/ADT/StringExtras.h"