[Clang] [NFC] Add an accessor for ASTUnit::CodeGenOpts (#172164)

There is currently no way to actually access `ASTUnit::CodeGenOpts`;
this is almost certainly an oversight, so this adds a getter for it.

Author: Sirraide

clang/include/clang/Frontend/ASTUnit.h

@@ -476,6 +476,11 @@ class ASTUnit {
     return *LangOpts;
   }
 
+  const CodeGenOptions &getCodeGenOpts() const {
+    assert(CodeGenOpts && "ASTUnit does not have codegen options");
+    return *CodeGenOpts;
+  }
+
   const HeaderSearchOptions &getHeaderSearchOpts() const {
     assert(HSOpts && "ASTUnit does not have header search options");
     return *HSOpts;