Synthesizing the definition of an implicit member is an AST modification, so notify...

Synthesizing the definition of an implicit member is an AST modification, so notify any mutation listeners of it. This fixes a crasher in chained PCH, where an implicit destructor in a PCH gets a definition in a chained PCH, which is then lost. However, any further use of the destructor would cause its definition to be regenerated in the final file, hiding the bug.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@130103 91177308-0d34-0410-b5e6-96231b3b80d8