[clang] [clang-repl] Skip CodeGen for top-level decls when diagnostics report errors (PR #169989)

[Anutosh Bhat via cfe-commits]

================
@@ -20,6 +20,9 @@ const char* c_null_str = 0; c_null_str
 "Hello, world"
 // CHECK-NEXT: ({{(const )?}}char[13]) "Hello, world"
 
+printf("Hello World\n");
+// expected-error {{call to undeclared library function 'printf' with type 'int (const char *, ...)' }}
+
 int x = 42; x
 // CHECK-NEXT: (int) 42
 
----------------
anutosh491 wrote:

Added a test here (can possibly open a new file if needed) but seems like an appropriate place.

On master 
```
anutosh491 at vv-nuc:/build/anutosh491/llvm-project/build/bin$ ./clang-repl   --Xcc=-x --Xcc=c
clang-repl> printf("Hello World\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("Hello World\n");
      | ^
input_line_1:1:1: note: include the header <stdio.h> or explicitly provide a declaration for 'printf'
error: Parsing failed.
clang-repl> int x = 42; x
Hello World
(int) 42
```
On branch
```
anutosh491 at vv-nuc:/build/anutosh491/llvm-project/build/bin$ ./clang-repl   --Xcc=-x --Xcc=c
clang-repl> printf("10\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("10\n");
      | ^
input_line_1:1:1: note: include the header <stdio.h> or explicitly provide a declaration for 'printf'
error: Parsing failed.
clang-repl> int x = 42; x
(int) 42
```

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