C front-end: reject invalid array declarators

[tautschnig]

• Reject VLA declarators with * outside function definitions
• Reject type qualifiers and storage class specifiers in array declarators outside function prototypes

According to the C standard (C99/C11):

• Variable length array declarators with * are only valid in function parameter declarations within function definitions (not declarations)
• Type qualifiers (like restrict, const, volatile) in array declarators are only valid in function parameter declarations
• Storage class specifiers (like static) in array declarators are only valid in function parameter declarations within function prototypes or definitions

These constructs are not valid in:

• File scope variable declarations
• Block scope variable declarations
• Any array declarator in non-parameter contexts

Fixes: #132

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