[clang] [clang][analyzer] Move checker alpha.unix.StdCLibraryFunctions out of alpha. (PR #66207)

[via cfe-commits]

================
@@ -1026,6 +1026,100 @@ Check for null pointers being passed as arguments to C string functions:
    return strlen(0); // warn
  }
 
+.. _unix-StdCLibraryFunctions:
+
+unix.StdCLibraryFunctions (C)
+"""""""""""""""""""""""""""""""""""
+Check for calls of standard library functions that violate predefined argument
+constraints. For example, it is stated in the C standard that for the ``int
+isalnum(int ch)`` function the behavior is undefined if the value of ``ch`` is
+not representable as unsigned char and is not equal to ``EOF``.
+
+.. code-block:: c
+
+  #define EOF -1
+  void test_alnum_concrete(int v) {
+    int ret = isalnum(256); // \
+    // warning: Function argument outside of allowed range
+    (void)ret;
+  }
+
+  void buffer_size_violation(FILE *file) {
+    enum { BUFFER_SIZE = 1024 };
+    wchar_t wbuf[BUFFER_SIZE];
+
+    const size_t size = sizeof(*wbuf);   // 4
+    const size_t nitems = sizeof(wbuf);  // 4096
+
+    // Below we receive a warning because the 3rd parameter should be the
+    // number of elements to read, not the size in bytes. This case is a known
+    // vulnerability described by the ARR38-C SEI-CERT rule.
+    fread(wbuf, size, nitems, file);
+  }
+
+You can think of this checker as defining restrictions (pre- and postconditions)
+on standard library functions. Preconditions are checked, and when they are
+violated, a warning is emitted. Post conditions are added to the analysis, e.g.
+that the return value must be no greater than 255.
+
+For example if an argument to a function must be in between 0 and 255, but the
+value of the argument is unknown, the analyzer will conservatively assume that
+it is in this interval. Similarly, if a function mustn't be called with a null
+pointer and the null value of the argument can not be proven, the analyzer will
+assume that it is non-null.
----------------
DonatNagyE wrote:

```suggestion
pointer and the analyzer cannot prove that it is null, then it will assume that
it is non-null.
```
"can not be proven" is stronger than "the analyzer cannot prove it"

https://github.com/llvm/llvm-project/pull/66207