[SyntaxTree][Synthesis] Fix: `deepCopy` -> `deepCopyExpandingMacros`.

There can be Macros that are tagged with `modifiable`. Thus verifying
`canModifyAllDescendants` is not sufficient to avoid macros when deep
copying.

We think the `TokenBuffer` could inform us whether a `Token` comes from
a macro. We'll look into that when we can surface this information
easily, for instance in unit tests for `ComputeReplacements`.

Differential Revision: https://reviews.llvm.org/D88034