[Clang][Sema] Fix invalid redefinition error in if/switch/for statement

[Clang][Sema] Fix invalid redefinition error in if/switch/for statement

Clang should no longer incorrectly diagnose a variable declaration inside of a
lambda expression that shares the name of a variable in a containing
if/while/for/switch init statement as a redeclaration.

After this patch, clang is supposed to accept code below:

void foo() {
  for (int x = [] { int x = 0; return x; }(); ;) ;
}

Fixes https://github.com/llvm/llvm-project/issues/54913

Differential Revision: https://reviews.llvm.org/D123840