[clang] [clang-repl] Drop CodeGen module when an input has parse errors (PR #169989)

[Anutosh Bhat via cfe-commits]

anutosh491 wrote:

So this fix discards the current CodeGen module whenever `Diags.hasErrorOccurred()`, **so IR from a failing input is dropped and cannot leak into the next cell**. After the patch in debug mode we can see the following !

```
anutosh491 at vv-nuc:/build/anutosh491/llvm-project/build/bin$ ./clang-repl --debug-only=clang-repl
compile-ptu 0: [TU=0x55556cf78168, M=0x55556cf72c60 (incr_module_0)]
execute-ptu 0: [TU=0x55556cf78168, M=0x55556cf72c60 (incr_module_0)]
clang-repl> printf("hi\n");
In file included from <<< inputs >>>:1:
input_line_1:1:1: error: use of undeclared identifier 'printf'
    1 | printf("hi\n");
      | ^~~~~~
error: Parsing failed.
clang-repl> #include <stdio.h>
compile-ptu 1: [TU=0x55556cfc7c08, M=0x55556cfbeee0 (incr_module_1)]
execute-ptu 1: [TU=0x55556cfc7c08, M=0x55556cfbeee0 (incr_module_1)]
clang-repl> printf("hi\n");
compile-ptu 2: [TU=0x55556d02cd68, M=0x55556cfca4e0 (incr_module_2)]
execute-ptu 2: [TU=0x55556d02cd68, M=0x55556cfca4e0 (incr_module_2)]
hi
clang-repl> 

anutosh491 at vv-nuc:/build/anutosh491/llvm-project/build/bin$ ./clang-repl --Xcc=-x --Xcc=c --Xcc=-std=c23 --debug-only=clang-repl
compile-ptu 0: [TU=0x55556cfb2b18, M=0x55556cf95a90 (incr_module_0)]
execute-ptu 0: [TU=0x55556cfb2b18, M=0x55556cf95a90 (incr_module_0)]
clang-repl> printf("hi\n");
In file included from <<< inputs >>>:1:
input_line_1:1:1: error: call to undeclared library function 'printf' with type
      'int (const char *, ...)'; ISO C99 and later do not support implicit function
      declarations [-Wimplicit-function-declaration]
    1 | printf("hi\n");
      | ^
input_line_1:1:1: note: include the header <stdio.h> or explicitly provide a
      declaration for 'printf'
error: Parsing failed.
clang-repl> #include <stdio.h> 
compile-ptu 1: [TU=0x55556cfbfb20, M=0x55556cf979a0 (incr_module_1)]
execute-ptu 1: [TU=0x55556cfbfb20, M=0x55556cf979a0 (incr_module_1)]
clang-repl> printf("hi\n");
compile-ptu 2: [TU=0x55556d00b5f8, M=0x55556cfc1790 (incr_module_2)]
execute-ptu 2: [TU=0x55556d00b5f8, M=0x55556cfc1790 (incr_module_2)]
hi
```

https://github.com/llvm/llvm-project/pull/169989