C#: Introduce Expr.getIntValue.

[aschackmull]

I noticed that the Expr.getValue().toInt() pattern was generating some poor performance due to lots of repeated string operations, so at first I just wanted to cache this to improve performance, but then I noticed that the pattern actually also yields results for string and char literals (e.g. "4" and '4' both yield 4), which is very unlikely to be intended in the various uses of this pattern. So besides improving some performance, this may also fix some bugs.

[Copilot]

Pull request overview

This PR introduces a new Expr.getIntValue() helper in the C# QL libraries to replace the common getValue().toInt() pattern, improving performance (via caching) and tightening semantics to only integral/enum-typed expressions.

Changes:

• Added cached Expr.getIntValue() in the core expression library.
• Updated a set of queries/libraries to use getIntValue() instead of getValue().toInt() (and in one place replaced a string comparison "0" with an integer comparison 0).

Show a summary per file

File	Description
csharp/ql/lib/semmle/code/csharp/exprs/Expr.qll	Adds the new cached getIntValue() API on Expr.
csharp/ql/src/Security Features/InsufficientKeySize.ql	Uses getIntValue() for key-size threshold checks.
csharp/ql/src/Likely Bugs/PossibleLossOfPrecision.ql	Uses getIntValue() for exact integer-division detection.
csharp/ql/src/Likely Bugs/MishandlingJapaneseEra.ql	Uses getIntValue() for date/era argument checks (including era == 0).
csharp/ql/src/Likely Bugs/BadCheckOdd.ql	Uses getIntValue() for parity-check constants and positivity checks.
csharp/ql/lib/semmle/code/csharp/frameworks/system/runtime/CompilerServices.qll	Uses getIntValue() for attribute constructor argument extraction.
csharp/ql/lib/semmle/code/csharp/dataflow/internal/rangeanalysis/ConstantUtils.qll	Uses getIntValue() for recognizing constant integer expressions.
csharp/ql/lib/semmle/code/csharp/controlflow/Guards.qll	Replaces local integer-constant parsing with getIntValue().
csharp/ql/lib/semmle/code/csharp/commons/Constants.qll	Uses getIntValue() when computing integral min/max for constant comparisons.
csharp/ql/lib/semmle/code/csharp/commons/ComparisonTest.qll	Uses getIntValue() for constant extraction in comparison-test logic.
csharp/ql/lib/semmle/code/csharp/Conversion.qll	Uses getIntValue() in constant conversion predicates.
csharp/ql/examples/snippets/integer_literal.ql	Updates snippet to demonstrate getIntValue().

Copilot's findings

• Files reviewed: 12/12 changed files
• Comments generated: 0

[michaelnebel]

LGTM!