[clang] Fix a crash on invalid auto.

Summary:
The crash is triggered on accessing a null InitExpr.

For group declaration, e.g. `auto c = a, &d = {a};`, what's happening:

1. each VarDecl is built separately during the parsing stage.
2. perform the semantic analysis (Sema::BuildDeclaratorGroup) to check
whether the type of the two VarDecl is the same, if not mark it as invalid.

in step 1, VarDecl c and d are built, both of them are valid (after D77395),
but d is without the InitExpr attached (under -fno-recovery-ast), crash
happens in step 2 when accessing the source range of d's InitExpr.

Reviewers: sammccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79473