[clang][Serialization] Stop including Frontend headers from Serialization (NFC) (#123140)

The Frontend library depends on Serialization. This is an explicit
dependency encoded in the CMake target. However, Serialization currently
has an implicit dependency on Frontend, as it includes one of its
headers. This is not reflected in the CMake build rules, but Bazel is
stricter so, in order to avoid a dependency cycle, it hackily declares
the Frontend headers as source files for Serialization.

Fortunately, the only Frontend header used by Serialization is
clang/Frontend/FrontendDiagnostic.h, which is a legacy header that just
includes clang/Basic/DiagnosticFrontend since
d076608d58d1ec55016eb747a995511e3a3f72aa, back in 2018.

This commit changes Serialization to use the underlying header from
Basic instead. Both Serialization and Frontend depend on Basic, so this
breaks the dependency cycle.

1 files changed, 1 insertions, 1 deletions

diff --git a/clang/lib/Serialization/GeneratePCH.cpp b/clang/lib/Serialization/GeneratePCH.cpp
index a3189bb..12751be 100644
--- a/clang/lib/Serialization/GeneratePCH.cpp
+++ b/clang/lib/Serialization/GeneratePCH.cpp
@@ -12,7 +12,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/AST/ASTContext.h"
-#include "clang/Frontend/FrontendDiagnostic.h"
+#include "clang/Basic/DiagnosticFrontend.h"
 #include "clang/Lex/HeaderSearch.h"
 #include "clang/Lex/HeaderSearchOptions.h"
 #include "clang/Lex/Preprocessor.h"