[BUG] Warnings in `cpp2util.h` and generated C++ code when compiling with a high warning level

[bluetarpmedia]

Describe the bug When compiling the regression-tests with a high warning level there are warnings in cpp2util.h as well as in some of the generated C++ code that cppfront produces.

To Reproduce

1. Edit the regression-tests\test-results\msvc-2022\run-tests-msvc-2022.bat file
2. Add -W4 to the cl.exe command on line 19
3. Run the run-tests-msvc-2022.bat file

I only did so for MSVC but I imagine adding -Wall etc to the scripts for Clang & GCC will show similar warnings.

Warnings in cpp2util.h

See the mixed-inspect-values.cpp.output file for some good examples.

Warnings about conversions and unreachable code

``` mixed-inspect-values.cpp ..\..\..\include\cpp2util.h(1376): warning C4244: 'argument': conversion from 'const _First' to 'int', possible loss of data with [ _First=double ] ..\..\..\include\cpp2util.h(1376): note: the template instantiation context (the oldest one first) is mixed-inspect-values.cpp2(10): note: see reference to function template instantiation 'void test>(const _T0 &)' being compiled with [ _T0=std::variant ] mixed-inspect-values.cpp2(35): note: see reference to function template instantiation 'bool cpp2::is>(const std::variant &,_T0 &&)' being compiled with [ _T0=in:: ] ..\..\..\include\cpp2util.h(1178): warning C4244: 'argument': conversion from 'const _T0' to 'int', possible loss of data with [ _T0=double ] ..\..\..\include\cpp2util.h(1178): note: the template instantiation context (the oldest one first) is mixed-inspect-values.cpp2(28): note: see reference to function template instantiation 'void test(const _T0 &)' being compiled with [ _T0=double ] mixed-inspect-values.cpp2(35): note: see reference to function template instantiation 'bool cpp2::is<_T0,in::>(const _T0 &,_T1 &&)' being compiled with [ _T0=double, _T1=in:: ] cpp2util.h(1616) : warning C4702: unreachable code cpp2util.h(1188) : warning C4702: unreachable code cpp2util.h(1570) : warning C4702: unreachable code ```

Some of the unreachable code warnings are fixed by PR #971

Warnings in generated C++ code

Here are the MSVC output files containing compiler warnings

``` mixed-captures-in-expressions-and-postconditions.cpp.output mixed-inspect-values-2.cpp.output mixed-inspect-values.cpp.output mixed-intro-example-three-loops.cpp.output mixed-postexpression-with-capture.cpp.output pure2-bugfix-for-unbraced-function-expression.cpp.output pure2-bugfix-for-variable-template.cpp.output pure2-enum.cpp.output pure2-inspect-generic-void-empty-with-variant-any-optional.cpp.output pure2-last-use.cpp.output pure2-more-wildcards.cpp.output pure2-print.cpp.output pure2-requires-clauses.cpp.output pure2-stdio.cpp.output ```

The warnings include things like:

warning C4389: '==': signed/unsigned mismatch
warning C4211: nonstandard extension used: redefined extern to static
warning C4996: 'fopen': This function or variable may be unsafe. Consider using fopen_s instead.
warning C4189: 'auto_1': local variable is initialized but not referenced

Some of them may be expected (e.g. using fopen) but others may warrant some investigation. The warning about auto_1 will be fixed by PR #969.

The redefined extern to static warning relates to this:

pure2-requires-clauses.cpp2 line 20:

v: <T> const T requires std::same_as<T, i32> = 0;

Lowered to:

template<typename T> 
CPP2_REQUIRES_ (std::same_as<T,cpp2::i32>) extern T const v;