[CMake][MS] libclang: Invalidate LLVM_EXPORTED_SYMBOL_FILE to suppress LNK4197.
[CMake][MS] libclang: Invalidate LLVM_EXPORTED_SYMBOL_FILE to suppress LNK4197.

Each functions is exported as "dllexport" in include/clang-c.
See also KB835326.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@199799 91177308-0d34-0410-b5e6-96231b3b80d8

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