Reland "[lldb][RPC] Upstream lldb-rpc-gen tool" (#146969)" Attempt 2 (#148996)

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

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