Second attempt at relanding the lldb-rpc-gen tool. This should fix 2
issues:

- An assert that was hitting when building on Linux. The assert would
hit in the server source emitter, specifically when attemping to
determine the storage size for a return type is that is a pointer, but
isn't a const char *, const char ** or void pointer.

The assert would hit when attempting to generate
SBAttachInfo::GetProcessPluginName, which returns a const char *
(meaning it shouldn't have been in the code block for the assert at
all). The reason that it was hitting the assert when generating this
function is that lldb_rpc_gen::TypeIsConstCharPtr was returning false
for this function even though it did return a const char *. This was
happening because when checking the return type for a const char *,
TypeIsConstCharPtr would only check that the underlying type was a
signed char. This failed on Linux (but was fine on Darwin), as the
underlying type also needs to be checked for being an unsigned char.

- Cross compiling support

The build for lldb-rpc-gen had no support for cross compiling and as
such, the sources generated for lldb-rpc-gen would get compiled too
early in phase 2 when cross compiling, before the Clang toolchain was
built and this led to an error when trying to include stdlib files. This
reland splits this build into 2 by building the tool first and then
compiling the sources in the second stage of the cross-compiled build.

Original PR Description:
This commit upstreams the lldb-rpc-gen tool, a ClangTool that generates
the LLDB RPC client and server interfaces. This tool, as well as LLDB
RPC itself is built by default. If it needs to be disabled, put
-DLLDB_BUILD_LLDBRPC=OFF in your CMake invocation.

https://discourse.llvm.org/t/rfc-upstreaming-lldb-rpc/85804

Original PR Link:
github.com/llvm/llvm-project/pull/138031

Reverts llvm/llvm-project#147417. Failing an assert:
`lldb-rpc-gen:
../llvm-project/lldb/tools/lldb-rpc/lldb-rpc-gen/server/RPCServerSourceEmitter.cpp:361:
void lldb_rpc_gen::RPCServerSourceEmitter::EmitMethodCallAndEncode(const
Method &): Assertion `Pos != MethodsWithPointerReturnTypes.end() &&
"Unable to determine the size of the return buffer"' failed.`

Relands the commit to upstream the lldb-rpc-gen tool in order to fix a
build failure on the linux remote bots. The reland adds the Clang
resource dir unconditionally to the invocation for the tool instead of
only adding it in the event that we're using a standalone build.

Original PR description:

This commit upstreams the lldb-rpc-gen tool, a ClangTool that generates
the LLDB RPC client and server interfaces. This tool, as well as LLDB
RPC itself is built by default. If it needs to be disabled, put
-DLLDB_BUILD_LLDBRPC=OFF in your CMake invocation.

https://discourse.llvm.org/t/rfc-upstreaming-lldb-rpc/85804

Original PR Link:

https://github.com/llvm/llvm-project/pull/138031

This commit upstreams the LLDB RPC server interface emitters. These
emitters generate the server-side API interfaces for RPC, which
communicate directly with liblldb itself out of process using the SB
API.

https://discourse.llvm.org/t/rfc-upstreaming-lldb-rpc/85804

Reverts llvm/llvm-project#138031. This is failing during the build phase
on the Ubuntu buildbot:
```
Error while processing /home/buildbot/worker/as-builder-9/lldb-remote-linux-ubuntu/llvm-project/lldb/include/lldb/API/SBWatchpoint.h.
[78/78] Processing file /home/buildbot/worker/as-builder-9/lldb-remote-linux-ubuntu/llvm-project/lldb/include/lldb/API/SBWatchpointOptions.h.
warning: unknown warning option '-Wno-maybe-uninitialized'; did you mean '-Wno-uninitialized'? [-Wunknown-warning-option]
warning: unknown warning option '-Wno-class-memaccess'; did you mean '-Wno-class-varargs'? [-Wunknown-warning-option]
warning: unknown warning option '-Wno-stringop-truncation'; did you mean '-Wno-format-truncation'? [-Wunknown-warning-option]
warning: unknown warning option '-Wno-maybe-uninitialized'; did you mean '-Wno-uninitialized'? [-Wunknown-warning-option]
warning: unknown warning option '-Wno-class-memaccess'; did you mean '-Wno-class-varargs'? [-Wunknown-warning-option]
warning: unknown warning option '-Wno-stringop-truncation'; did you mean '-Wno-format-truncation'? [-Wunknown-warning-option]
In file included from /home/buildbot/worker/as-builder-9/lldb-remote-linux-ubuntu/llvm-project/lldb/include/lldb/API/SBWatchpointOptions.h:12:
In file included from /home/buildbot/worker/as-builder-9/lldb-remote-linux-ubuntu/llvm-project/lldb/include/lldb/API/SBDefines.h:12:
In file included from /home/buildbot/worker/as-builder-9/lldb-remote-linux-ubuntu/llvm-project/lldb/include/lldb/lldb-defines.h:12:
In file included from /home/buildbot/worker/as-builder-9/lldb-remote-linux-ubuntu/llvm-project/lldb/include/lldb/lldb-types.h:12:
/home/buildbot/worker/as-builder-9/lldb-remote-linux-ubuntu/llvm-project/lldb/include/lldb/lldb-enumerations.h:12:10: fatal error: 'cstdint' file not found
   12 | #include <cstdint>
      |          ^~~~~~~~~
```

This commit upstreams the `lldb-rpc-gen` tool, a ClangTool that
generates the LLDB RPC client and server interfaces. This tool, as well
as LLDB RPC itself is built by default. If it needs to be disabled, put
-DLLDB_BUILD_LLDBRPC=OFF in your CMake invocation.

https://discourse.llvm.org/t/rfc-upstreaming-lldb-rpc/85804