Start fleshing out ASTMutationListener; notify when a tag definition is completed.

In that case a chained PCH will record the updates to the DefinitionData pointer of forward references.
If a forward reference mutated into a definition re-write it into the chained PCH, this is too big of a change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@117239 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed