[BUG][C++] Avoid using plain underscore when escaping reserved words

Created by: alexweav

Fixes #5268 (closed) as described in the issue. Are there any better preferred ways of escaping this?

Ran the generator against the file given in the issue, and verified that the output now compiled using MSVC-14. Repeated below, for ease of use:

    openapi: 3.0.0
    info:
      title: Inline API
      version: 0.0.1
    paths:
      /things:
        get:
          summary: Returns a list of things.
          parameters:
            - in: query
              name: inline
              schema:
                type: boolean
              description: Matches a keyword in C++.
          responses:
            '200':    # status code
              description: A JSON array of things.
              content:
                application/json:
                  schema: 
                    type: array
                    items: 
                      type: string

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