C grammar: introduce attribute_specifier

[kroening]

This is a preparatory step for introducing C2x-style attribute specifiers.

This also makes it easier to introduce other, custom attributes, say for
code contracts.

• 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/
• n/a 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

Merging #5945 (fbe9519) into develop (316ae8b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #5945   +/-   ##
========================================
  Coverage    73.56%   73.56%           
========================================
  Files         1431     1431           
  Lines       155291   155291           
========================================
  Hits        114243   114243           
  Misses       41048    41048           

Impacted Files	Coverage Δ
src/ansi-c/parser.y	77.77% <100.00%> (ø)

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