Parser ignores RegEx in Alternation with RegEx and FixedString

[anna-riesch]

Describe the bug

• Alternation with RegEx and FixedString
• Tabcompletion only completes Fixed string

Example for completing ./gwhisper <Tab>:

-h
--complete
--rpcTimeoutMilliseconds=<FixedString>
...

To Reproduce
Add an Alternation-GrammarElement in GrammarConstruction.cpp, containing a FixedString and a RegEx, e.g.:

GrammarElement *timeout = f_grammarPool.createElement<Concatenation>(); timeout->addChild(f_grammarPool.createElement<FixedString>("--rpcTimeoutMilliseconds=", "rpcTimeout"));
GrammarElement *timeoutTime = f_grammarPool.createElement<Alternation>();
 timeout->addChild(timeoutTime);
timeoutTime->addChild(f_grammarPool.createElement<RegEx>("[0-9]+", "rpcTimeoutInMs"));
timeoutTime->addChild(f_grammarPool.createElement<FixedString>("None", "manualInfiniteTimeout"));
optionsalt->addChild(timeout);

Expected behavior
When completing, all options of an alternation are shown, allowing the user to choose between using the FixedString or his own input.
e.g.
./gwhisper --rpcTimeoutMilliseconds=<Tab>
returns

-h
--complete
--rpcTimeoutMilliseconds=
...

and then typing
./gwhisper --rpcTimeoutMilliseconds=<Tab>
should return the options:
--rpcTimeoutMilliseconds=<RegEx> --rpcTimeoutMilliseconds=<FixedString>

Provide details about your environment

• Operating System version: Fedora 35
• gwhisper --version output: v4.0.0-9
• gRPC/protobuf version:
• Compiler version (if related to build):
• CMake version (if related to build):

Additional context
Add any other context about the problem here.