Move verification of Sema::MaximumAlignment to a .cpp file

Saves these transitive includes:
diff --git a/clang/lib/CodeGen/CGDecl.cpp b/clang/lib/CodeGen/CGDecl.cpp
index 589bd20..2946259 100644
--- a/clang/lib/CodeGen/CGDecl.cpp
+++ b/clang/lib/CodeGen/CGDecl.cpp
@@ -31,6 +31,7 @@
 #include "clang/Basic/SourceManager.h"
 #include "clang/Basic/TargetInfo.h"
 #include "clang/CodeGen/CGFunctionInfo.h"
+#include "clang/Sema/Sema.h"
 #include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/GlobalVariable.h"
@@ -40,6 +41,9 @@
 using namespace clang;
 using namespace CodeGen;
 
+static_assert(clang::Sema::MaximumAlignment <= llvm::Value::MaximumAlignment,
+              "Clang max alignment greater than what LLVM supports?");
+
 void CodeGenFunction::EmitDecl(const Decl &D) {
   switch (D.getKind()) {
   case Decl::BuiltinTemplate: