[Sema] Use lexical DC for friend functions when getting constraint instantiation args (#77552)

Fixes a crash where the template argument depth computed in the semantic
context for a friend FunctionDecl with a constrained parameter is
compared against arguments in the lexical context for the purpose of
checking if the constraint depends on enclosing template parameters.

Since getTemplateInstantiationArgs in this case follows the semantic DC
for friend FunctionDecls, the resulting depth is incorrect and trips an
assertion.

Fixes #75426