[clang] [LifetimeSafety] Add bailout for large CFGs (PR #170444)

[via cfe-commits]

DEBADRIBASAK wrote:

Output with `cfg-num-block-threshold=5`:

```
==========================================
       Lifetime Analysis CFG Block Sizes:
==========================================
Function: llvm::itanium_demangle::starts_with
Number of CFG Blocks: 5
  Block B4: Number of elements = 0
  Block B3: Number of elements = 5
  Block B2: Number of elements = 11
  Block B1: Number of elements = 4
  Block B0: Number of elements = 0
==========================================
       Lifetime Analysis CFG Block Sizes:
==========================================
Function: llvm::itanium_demangle::starts_with
Number of CFG Blocks: 5
  Block B4: Number of elements = 0
  Block B3: Number of elements = 9
  Block B2: Number of elements = 4
  Block B1: Number of elements = 24
  Block B0: Number of elements = 0
==========================================
       Lifetime Analysis CFG Block Sizes:
==========================================
Function: isRustEncoding
Number of CFG Blocks: 3
  Block B2: Number of elements = 0
  Block B1: Number of elements = 12
  Block B0: Number of elements = 0
==========================================
       Lifetime Analysis CFG Block Sizes:
==========================================
Function: isDLangEncoding
Number of CFG Blocks: 3
  Block B2: Number of elements = 0
  Block B1: Number of elements = 12
  Block B0: Number of elements = 0
```

https://github.com/llvm/llvm-project/pull/170444