Include cleaner prefers <assert.h> over <cassert> for assert() in C++ files

[sam-mccall]

TL;DR: assert.h is getting the PreferredHeader signal due to name match.

// test.cpp
#include <bits/stdc++.h> // assuming libstdc++
void x() { assert(42); }

Clangd correctly produces the diagnostic No header providing "assert" is directly included.
The suggested fix is to insert #include <assert.h>, but we should prefer <cassert>.

<cassert> is listed first in StdSymbolMap.inc, so would in general be preferred, but because the symbol name matches the header name the PreferredHeader signal kicks in.

[llvmbot]

@llvm/issue-subscribers-clang-include-cleaner

[hokein]

The <cassert> should have the PreferredHeader hint applied as well as it is the first candidate, https://github.com/llvm/llvm-project/blob/main/clang-tools-extra/include-cleaner/lib/FindHeaders.cpp#L105-L108.

The both headers have the same ranking signal, but the <cassert> is the first one in the Headers sequence, we should select it. However the sequence is re-arranged when we perform the deduplications here, we sort the array by name (so assert.h become the first one!)