[clang-tools-extra] [clang-tidy] Add llvm-regex check (PR #207407)

[Donát Nagy via cfe-commits]

================
@@ -0,0 +1,36 @@
+.. title:: clang-tidy - llvm-invalid-regex-pattern
+
+llvm-invalid-regex-pattern
+==========================
+
+Detects malformed regex patterns defined in a single string literal
+to catch mistakes at compile time.
+It detects these string literals if they are defined in the regex constructor
+with a string literal, or stored in one of these non mutable container:
+
+- ``const std::string``
+- ``const char*``
+- ``const llvm::StringRef``
+- ``std::string_view``
+
+In the event that the patterns are stored as a class member, the check verifies
+the initialization value, if defined, even if its overwritten by the constructor.
+
+Example of detection:
+
+.. code-block:: c++
+
+    const std::string my_regex_pattern("[0-8"); // invalid regex pattern
+    llvm::Regex my_regex(my_regex_pattern);
+
+Example of member data being overwritten:
+
+.. code-block:: c++
+
+  class foo{
+  public:
+    foo(llvm::StringRef regex_ptrn) : regex_prtn(regex_prtn){};
+    const llvm_StringRef regex_prtn = "("; // invalid regex pattern
+  };
+  foo bar("[0-9]");
+  llvm::Regex my_regex(bar.regex_prtn);
----------------
NagyDonat wrote:

```suggestion
    foo(llvm::StringRef regex_pat) : regex_p(regex_pat){};
    const llvm_StringRef regex_pat = "("; // invalid regex pattern
  };
  foo bar("[0-9]");
  llvm::Regex my_regex(bar.regex_pat);
```
In some locations you wrote "prtn" instead of "ptrn" so I propose using an abbreviation that is easier to pronounce/read :). (Feel free to pick any other variable name that you prefer.)

https://github.com/llvm/llvm-project/pull/207407