[clangd] TargetFinder: Fix assert-crash on TemplateExpansion args.

Previously we would call getAsTemplate() when kind == TemplateExpansion,
which triggers an assertion. The call is now replaced with
getAsTemplateOrTemplatePattern(), which is exactly the same as
getAsTemplate(), except it allows calls when kind == TemplateExpansion.

No change in behavior for no-assert builds.

Differential Revision: https://reviews.llvm.org/D111648