Remove unused Endian.h includes, NFC

Mainly avoids including Host.h everywhere:

$ diff -u <(sort thedeps-before.txt) <(sort thedeps-after.txt) \
    | grep '^[-+] ' | sort | uniq -c | sort -nr
   3141 - /usr/local/google/home/rnk/llvm-project/llvm/include/llvm/Support/Host.h
diff --git a/clang/unittests/AST/StructuralEquivalenceTest.cpp b/clang/unittests/AST/StructuralEquivalenceTest.cpp
index 8e46752..493b847 100644
--- a/clang/unittests/AST/StructuralEquivalenceTest.cpp
+++ b/clang/unittests/AST/StructuralEquivalenceTest.cpp
@@ -1,8 +1,9 @@
 #include "clang/AST/ASTContext.h"
-#include "clang/ASTMatchers/ASTMatchers.h"
 #include "clang/AST/ASTStructuralEquivalence.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
 #include "clang/Frontend/ASTUnit.h"
 #include "clang/Tooling/Tooling.h"
+#include "llvm/Support/Host.h"
 
 #include "Language.h"
 #include "DeclMatcher.h"