Implement modernize-use-constraints

[ccotter]

Summary

Add new check to replace enable_if with C++20 constraints

[ccotter]

For the sake of demonstration, https://github.com/llvm/llvm-project/commit/9c556ce59edf5a4293d4497d5815544afc0eb878 is the result of running this tool on all headers under clang/include/clang and llvm/include/llvm.

[ccotter]

Overall, we could eventually upgrade code in three stages, each a separate reusable check.

1. enable_if -> requires clauses
2. replace the non _v templates to the _v variants is_same -> is_same_v or the equivalent concept same_as
3. replace requires clause on declarations to be template type constraint (replace template <typename T> void foo() requires std::same_as<T, int> void foo() {} to template <std::same_as<int> T> void foo() {}

[njames93]

In D141892#4057722, @ccotter wrote:

2. replace the non _v templates to the _v variants is_same -> is_same_v or the equivalent concept same_as

See D137302

[ccotter]

In D141892#4058273, @njames93 wrote:

In D141892#4057722, @ccotter wrote:

2. replace the non _v templates to the _v variants is_same -> is_same_v or the equivalent concept same_as

See D137302

Awesome!

[ccotter]

• Add fno-delayed-template-parsing

[ccotter]

• Use nested namespaces

[ccotter]

Rebase + Simplify match logic

[ccotter]

This is more or less ready for review (not planning on making any further changes; there are more features to be added, but I was thinking of handling those in follow up changesets). I know it's a relatively large review, but let me know if anyone can take a first pass. Thanks!

[ccotter]

rebase