Add 'clangSerialization' to CMakeLists (fix downstream link error).

[tylanphear]

The following link error occurs in downstream after 1bda00e:

ld: error: undefined symbol: clang::PCHContainerOperations::PCHContainerOperations()
>>> referenced by opencl_clang.cpp
>>>               _deps/opencl-clang-build/CMakeFiles/common_clang.dir/opencl_clang.cpp.o:(Compile)
icpx: error: linker command failed with exit code 1 (use -v to see invocation)

This is similar to the link error seen in
https://reviews.llvm.org/D157078, which was resolved in the same way
(https://reviews.llvm.org/rG36daf3532d91bb3e61d631edceea77ebb8417801).

This patch just adds 'clangSerialization' to the link libraries to
resolve the issue.

[tylanphear]

This fix is also urgent to unblock the downstream build. Pinging @hewj03, @haonanya for review.

[wenju-he]

LGTM

[wenju-he]

@tylanphear I just take a look at link.txt and it show opencl-clang is linking libclangSerialization.a
The root cause is probably that opencl-clang doesn't depends on clangSerialization target and opencl-clang is linking its dependencies before clangSerialization is built. So this build error could happen to other clang static libraries. 1bda00e forgot to use add_dependencies.
I suggest to revert 1bda00e first, WDYT?

[wwXing0]

@tylanphear I just take a look at link.txt and it show opencl-clang is linking libclangSerialization.a The root cause is probably that opencl-clang doesn't depends on clangSerialization target and opencl-clang is linking its dependencies before clangSerialization is built. So this build error could happen to other clang static libraries. 1bda00e forgot to use add_dependencies. I suggest to revert 1bda00e first, WDYT?

reverts commit 1bda00e by #476

[tylanphear]

Closing in favor of #476

[wenju-he]

after discussion with @zhaomaosu, we think this pr is probably the right fix although it is still unclear why clangSerialization is needed twice in link.txt.
clangSerialization is also added to some clang tools CMakeLists.txt
So I'll revert #476 and then re-open this PR.

[wenju-he]

@tylanphear branch tylanphear:fix_downstream_link_error is deleted, could you please restore the branch and re-open this pr?

[tylanphear]

Reopening as requested by @wenju-he.