[Clang] Remove the special-casing for RequiresExprBodyDecl in BuildResolvedCallExpr() after fd87d765 (#111277)

The special-casing for RequiresExprBodyDecl caused a regression, as
reported in #110785.

The original fix for #84020 has been superseded by fd87d765, which
establishes a `DependentScopeDeclRefExpr` instead of a
`CXXDependentScopeMemberExpr` for the case in issue. So the spurious
diagnostic in #84020 would no longer occur.

This also merges the test for #84020 together with that for #110785 into
clang/test/SemaTemplate/instantiate-requires-expr.cpp.

No release note because I think this merits a backport.

Fixes #110785

(cherry picked from commit 8c1547055eaf65003f3e6fd024195f4926ff2356)