[libclang] Add missing dllexport annotation (#147108)
All other declarations of clang-c already have CINDEX_LINKAGE.
This missing annotation causes a linker error when building
`tools/clang/unittests/libclang/CrashTests/libclangCrashTests.exe` for
the Cygwin target.
On the regular Win32 target, this issue went unnoticed because the
entire libclang gtest-based testsuite is currently disabled for that
platform.
