Improve IsValidNewMemberName accuracy

[sharwell]

This update follows the discussion in #1849.

[codecov-io]

Current coverage is 79.28%

Merging #1853 into master will decrease coverage by -0.07% as of b11498a

@@            master   #1853   diff @@
======================================
  Files          554     554       
  Stmts        34242   34258    +16
  Branches      9484    9491     +7
  Methods                          
======================================
- Hit          27173   27161    -12
- Partial       5512    5519     +7
- Missed        1557    1578    +21

Review entire Coverage Diff as of b11498a

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[vweijsters]

👍

[sharwell]

@vweijsters I didn't mean to include the change for enum members in this pull request. I removed that commit.