[Clang] Fix dependent local class instantiation bugs (#134038)

This patch fixes two long-standing bugs that prevent Clang from
instantiating local class members inside a dependent context. These bugs
were introduced in commits 21eb1af469c3 and 919df9d75a.

21eb1af469c3 introduced a concept called eligible methods such that it
did an attempt to skip past ineligible method instantiation when
instantiating class members. Unfortunately, this broke the instantiation
chain for local classes - getTemplateInstantiationPattern() would fail
to find the correct definition pattern if the class was defined within a
partially transformed dependent context.

919df9d75a introduced a separate issue by incorrectly copying the
DeclarationNameInfo during function definition instantiation from the
template pattern, even though that DNI might contain a transformed
TypeSourceInfo. Since that TSI was already updated when the declaration
was instantiated, this led to inconsistencies. As a result, the final
instantiated function could lose track of the transformed declarations,
hence we crash: https://compiler-explorer.com/z/vjvoG76Tf.

This PR corrects them by

1. Removing the bypass logic for method instantiation. The eligible flag
is independent of instantiation and can be updated properly afterward,
so skipping instantiation is unnecessary.

2. Carefully handling TypeSourceInfo by creating a new instance that
preserves the pattern's source location while using the already
transformed type.

Fixes https://github.com/llvm/llvm-project/issues/59734
Fixes https://github.com/llvm/llvm-project/issues/132208