[CIR][CIRGen] Move CIRGen types into clang::CIRGen (#1082)

#1025 explains why we want to move
the CIR dialect from the `mlir::cir` to the `cir` namespace. To avoid
overloading the `cir` namespace too much afterwards, move all symbols
whose equivalents live inside the `clang::CodeGen` namespace to a new
`clang::CIRGen` namespace, so that we match the original CodeGen's
structure more closely.

There's some symbols that live under `clang/include/clang/CIR` whose
equivalents live in `clang/lib/CodeGen` and are in the `clang::CodeGen`
namespace. We have these symbols in a common location since they're also
used by lowering, so I've also left them in the `cir` namespace. Those
symbols are:
- AArch64ABIKind
- ABIArgInfo
- FnInfoOpts
- TypeEvaluationKind
- X86AVXABILevel

This is a pretty large PR out of necessity. To make it slightly more
reviewable, I've split it out into three commits (which will be squashed
together when the PR lands):
- The first commit manually switches places to the `clang::CIRGen`
  namespace. This has to be manual because we only want to move things
  selectively.
- The second commit adjusts namespace prefixes to make builds work. I
  ran https://gist.github.com/smeenai/f4dd441fb61c53e835c4e6057f8c322f
  to make this change. The script is idempotent, and I added
  substitutions one at a time and reviewed each one afterwards (using
  `git diff --color-words=.`) to ensure only intended changes were being
  made.
- The third commit runs `git clang-format`.

Because I went one-by-one with all my substitutions and checked each one
afterwards, I'm pretty confident in the validity of all the changes
(despite the size of the PR).

Author: smeenai

clang/include/clang/CIR/CIRGenerator.h

@@ -34,12 +34,14 @@ namespace clang {
 class ASTContext;
 class DeclGroupRef;
 class FunctionDecl;
-} // namespace clang
 
-namespace cir {
+namespace CIRGen {
 class CIRGenModule;
 class CIRGenTypes;
+} // namespace CIRGen
+} // namespace clang
 
+namespace cir {
 class CIRGenerator : public clang::ASTConsumer {
   virtual void anchor();
   clang::DiagnosticsEngine &Diags;
@@ -70,7 +72,7 @@ class CIRGenerator : public clang::ASTConsumer {
 
 protected:
   std::unique_ptr<mlir::MLIRContext> mlirCtx;
-  std::unique_ptr<CIRGenModule> CGM;
+  std::unique_ptr<clang::CIRGen::CIRGenModule> CGM;
 
 private:
   llvm::SmallVector<clang::FunctionDecl *, 8> DeferredInlineMemberFuncDefs;

clang/lib/CIR/CodeGen/ABIInfo.h

@@ -11,9 +11,8 @@
 
 #include "clang/AST/Type.h"
 
-namespace cir {
+namespace clang::CIRGen {
 
-class ABIArgInfo;
 class CIRGenCXXABI;
 class CIRGenFunctionInfo;
 class CIRGenTypes;
@@ -42,6 +41,6 @@ class ABIInfo {
   bool isPromotableIntegerTypeForABI(clang::QualType Ty) const;
 };
 
-} // namespace cir
+} // namespace clang::CIRGen
 
 #endif