[NFC][CLANG] Fix nullptr dereference issue in DeduceTemplateArgumentsByTypeMatch()

DeduceTemplateArgumentsByTypeMatch() returns null value which is dereferenced without checking since getAsIncompleteArrayType() returns nullptr and we are dereferencing null pointer value for S.Context->getAsIncompleteArrayType(P) when calling getElementType().

This patch adds an assert.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D151529