[lldb][NFC] Rename ClangASTContext to TypeSystemClang

Summary:
This commit renames ClangASTContext to TypeSystemClang to better reflect what this class is actually supposed to do
(implement the TypeSystem interface for Clang). It also gets rid of the very confusing situation that we have both a
`clang::ASTContext` and a `ClangASTContext` in clang (which sometimes causes Clang people to think I'm fiddling
with Clang's ASTContext when I'm actually just doing LLDB work).

I also have plans to potentially have multiple clang::ASTContext instances associated with one ClangASTContext so
the ASTContext naming will then become even more confusing to people.

Reviewers: #lldb, aprantl, shafik, clayborg, labath, JDevlieghere, davide, espindola, jdoerfert, xiaobai

Reviewed By: clayborg, labath, xiaobai

Subscribers: wuzish, emaste, nemanjai, mgorny, kbarton, MaskRay, arphaman, jfb, usaxena95, jingham, xiaobai, abidh, JDevlieghere, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D72684
diff --git a/lldb/source/API/SystemInitializerFull.cpp b/lldb/source/API/SystemInitializerFull.cpp
index 28c3b3b..f91f3d5 100644
--- a/lldb/source/API/SystemInitializerFull.cpp
+++ b/lldb/source/API/SystemInitializerFull.cpp
@@ -22,7 +22,7 @@
 #include "lldb/Host/Host.h"
 #include "lldb/Initialization/SystemInitializerCommon.h"
 #include "lldb/Interpreter/CommandInterpreter.h"
-#include "lldb/Symbol/ClangASTContext.h"
+#include "lldb/Symbol/TypeSystemClang.h"
 #include "lldb/Utility/Timer.h"
 
 #include "Plugins/ABI/MacOSX-arm/ABIMacOSX_arm.h"
@@ -205,7 +205,7 @@
   llvm::InitializeAllTargetMCs();
   llvm::InitializeAllDisassemblers();
 
-  ClangASTContext::Initialize();
+  TypeSystemClang::Initialize();
 
 #define LLVM_TARGET(t) LLDB_PROCESS_ ## t(Initialize)
 #include "llvm/Config/Targets.def"
@@ -298,7 +298,7 @@
   // Terminate and unload and loaded system or user LLDB plug-ins
   PluginManager::Terminate();
 
-  ClangASTContext::Terminate();
+  TypeSystemClang::Terminate();
 
   ArchitectureArm::Terminate();
   ArchitectureMips::Terminate();