[clang] Fix crash and improve diagnostics for void lambda returns (PR #188904)

[via cfe-commits]

Sirraide wrote:

@Ko496-glitch Just tested this, and for me the test _does_ fail when I run it locally:
```console
$ ./out/bin/clang -cc1 ../clang/test/CXX/expr/expr.prim/expr.prim.lambda/p4.cpp -std=c++11 -verify
error: 'expected-error' diagnostics seen but not expected: 
  File ../clang/test/CXX/expr/expr.prim/expr.prim.lambda/p4.cpp Line 21: void lambda should not return a value
1 error generated.
```

https://github.com/llvm/llvm-project/pull/188904