Highlighting Indexer: Check for duplicate itemDatas

Summary:
Check for duplicate itemData entries during indexing.

Currently, there are the following duplicates:
AttributeChecker::processElement: "data/syntax/haml.xml" itemData duplicate: "Symbol"
AttributeChecker::processElement: "data/syntax/haml.xml" itemData duplicate: "String"
AttributeChecker::processElement: "data/syntax/haml.xml" itemData duplicate: "Keyword"
AttributeChecker::processElement: "data/syntax/ilerpg.xml" itemData duplicate: "Float"
AttributeChecker::processElement: "data/syntax/ilerpg.xml" itemData duplicate: "Float"
AttributeChecker::processElement: "data/syntax/rhtml.xml" itemData duplicate: "Error"
AttributeChecker::processElement: "data/syntax/systemverilog.xml" itemData duplicate: "Data Type Keyword"
AttributeChecker::processElement: "data/syntax/systemverilog.xml" itemData duplicate: "Preprocessor Keyword"
AttributeChecker::processElement: "data/syntax/systemverilog.xml" itemData duplicate: "System Task Keyword"
AttributeChecker::processElement: "data/syntax/systemverilog.xml" itemData duplicate: "Drive/Charge Strength Keyword"
AttributeChecker::processElement: "data/syntax/systemverilog.xml" itemData duplicate: "Gate Instantiation Keyword"
AttributeChecker::processElement: "data/syntax/xul.xml" itemData duplicate: "Comment"

Test Plan: make && make test

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D9129