[clang] [clang] increase default constexpr step limit (PR #143785)

[via cfe-commits]

================
@@ -3992,7 +3992,7 @@ Controlling implementation limits
   Sets the limit for the number of full-expressions evaluated in a single
   constant expression evaluation. This also controls the maximum size
   of array and dynamic array allocation that can be constant evaluated.
-  The default is 1048576.
+  The default is 20000000.
----------------
Sirraide wrote:

```suggestion
  The default is 20 000 000.
```
Maybe format it like this because this amount of zeroes is giving me a headache.

https://github.com/llvm/llvm-project/pull/143785