[clang] [NFC][Docs] Documenting __builtin_cpu_supports. (PR #84098)

[Jon Roelofs via cfe-commits]

================
@@ -2799,6 +2799,34 @@ counter's true frequency will need to be provided by the user.
 
 Query for this feature with ``__has_builtin(__builtin_readsteadycounter)``.
 
+``__builtin_cpu_supports``
+--------------------------
+
+**Syntax**:
+
+.. code-block:: c++
+
+  int __builtin_cpu_supports(const char *features);
+
+**Example of Use:**:
+
+.. code-block:: c++
+
+  if (__builtin_cpu_supports("sve"))
+    sve_code();
+
+**Description**:
+
+The ``__builtin_cpu_supports`` function detects at runtime if target CPU
+supports features specified in string argument. It returns positive integer
+if all features are supported and 0 otherwise. Names of features and format is
+target specific. For example on AArch64 features are combined using ``+`` like
+this ``__builtin_cpu_supports("flagm+sha3+lse+rcpc2+fcma+memtag+bti+sme2")``.
+If feature name is not supported or format is wrong, compiler will issue a
+warning and replace builtin by constant 0.
+
+Query for this feature with ``__has_builtin(__builtin_cpu_supports)``.
----------------
jroelofs wrote:

```suggestion
The ``__builtin_cpu_supports`` function detects at runtime if the target CPU
supports features specified in string argument. It returns a positive integer
if all features are supported and 0 otherwise. Names of features and format are
target specific. For example on AArch64 features are combined using ``+`` like
this ``__builtin_cpu_supports("flagm+sha3+lse+rcpc2+fcma+memtag+bti+sme2")``.
If a feature name is not supported or the format is wrong, the compiler will issue a
warning and replace builtin by the constant 0.

Query for this feature with ``__has_builtin(__builtin_cpu_supports)``.
```

https://github.com/llvm/llvm-project/pull/84098