Reduce Clang header exposure in check_unit.h (#7509)

Remove clang/Frontend/CompilerInvocation.h from check_unit.h and forward
declare clang::CompilerInvocation instead to reduce header exposure.

Add an explicit include of CompilerInvocation.h to check_unit.cpp.

Assisted-by: Antigravity with Gemini

toolchain/check/check_unit.cpp

@@ -9,6 +9,7 @@
 #include <tuple>
 #include <utility>
 
+#include "clang/Frontend/CompilerInvocation.h"
 #include "clang/Sema/Sema.h"
 #include "common/growing_range.h"
 #include "common/pretty_stack_trace_function.h"

toolchain/check/check_unit.h

@@ -5,7 +5,6 @@
 #ifndef CARBON_TOOLCHAIN_CHECK_CHECK_UNIT_H_
 #define CARBON_TOOLCHAIN_CHECK_CHECK_UNIT_H_
 
-#include "clang/Frontend/CompilerInvocation.h"
 #include "common/map.h"
 #include "llvm/ADT/SmallVector.h"
 #include "toolchain/check/check.h"
@@ -14,6 +13,10 @@
 #include "toolchain/parse/node_ids.h"
 #include "toolchain/sem_ir/ids.h"
 
+namespace clang {
+class CompilerInvocation;
+}
+
 namespace Carbon::Check {
 
 struct UnitAndImports;