C front-end: place requires and ensures in designated scope

[tautschnig]

These contract clauses may introduce new symbols, which must not conflict with ones declared in the body of the function.

Fixes: #8337

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.29%. Comparing base (3dcc1c7) to head (f008664).

Files	Patch %	Lines
src/ansi-c/parser.y	88.88%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8380      +/-   ##
===========================================
- Coverage    78.32%   78.29%   -0.04%     
===========================================
  Files         1726     1726              
  Lines       188494   188583      +89     
  Branches     18249    18249              
===========================================
+ Hits        147647   147656       +9     
- Misses       40847    40927      +80     

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