[clang] Move the definition of ASTDiff (NFC)

This patch moves the definition of ASTDiff later within the header
file.

Without this patch, the header depends on the forward decalrations of
SyntaxTree and ComparisonOptions from another header file, which is
not desirable.  Since SyntaxTree and ComparisonOptions are defined in
ASTDiff.h, we can move the definition of ASTDiff later and stop
relying on the forward declarations from another header file.