RFR: 8286647: JFR: Build failure when C1 or C2 is disabled after JDK-8282420

[Jie Fu]

On Thu, 12 May 2022 14:25:53 GMT, Markus Grönlund <mgronlun at openjdk.org> wrote:

> Ok, the function definitions in the .cpp will also have to be conditionalised. Is the rest necessary? Forward declarations are just symbolic pointers until they reach their definition.

At the beginning, I didn't guard the forward declarations and the build still passed on my Linux/x64.

#ifdef COMPILER1
class GraphBuilder;
#endif

#ifdef COMPILER2
class Parse;
#endif


However, I'm not sure if all the compilers would pass without the  two `#ifdef` above.
So to make it to be safe enough, I just added them.

Are you sure it's safe to remove them? 
Thanks.

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PR: https://git.openjdk.java.net/jdk/pull/8680