Only inject file-level instr. for first pragma in file

[cgewecke]

#769

Flattened contracts often have multiple pragma statements. Since solc 0.7.x we've injected file-level instrumentation to handle file-level function declarations but this has been happening each time the parser hits a pragma resulting in:

"DeclarationError: Function with same name and parameter types defined twice.\n --> test.sol:9:1:
   |
9  | function c_true5c65fa93(bytes8 c__5c65fa93) pure returns (bool){ return true; }
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Note: Other declaration is here...
"message": "Function with same name and parameter types defined twice.",

PR adds some logic to ensure we only do this once. Also removing diamond inheritance test which didn't repro error in #769.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (351696e) 96.36% compared to head (ab272e7) 96.36%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #865   +/-   ##
=======================================
  Coverage   96.36%   96.36%           
=======================================
  Files          19       19           
  Lines        1017     1019    +2     
=======================================
+ Hits          980      982    +2     
  Misses         37       37           

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