[clang][AST] TextNodeDumper should not evaluate the initializer of constexpr variable...

[clang][AST] TextNodeDumper should not evaluate the initializer of constexpr variable declaration when it has a dependent type

`TextNodeDumper` enabed through `-ast-dump` flag should not evlauate the initializer when it visits a constexpr `VarDecl` node if it has a dependent type.

I found a crashing case fixed by this change and added it as a test case.
`template <typename T> constexpr T call_init(0);`
Link: https://godbolt.org/z/3bG9Pjj5E
This is a fix for the regression caused by D146358

Differential Revision: https://reviews.llvm.org/D151033