[modules] Fix crash in call to `FunctionDecl::setPure()`

In some cases, when deserializing a `CXXMethodDecl` of a `CXXSpecializationTemplateDecl`,
the call to `FunctionDecl::setPure()` happens before the `DefinitionData` member has been
populated (which appears to happen lower down in a `mergeRedeclarable` call), causing a
crash (https://reviews.llvm.org/P8228).

This diff fixes this by deferring the `FunctionDecl::setPure()` till after the `DefinitionData` has
been filled in.

Reviewed By: lxfind

Differential Revision: https://reviews.llvm.org/D86853