[libclang] Remove the 'extern "C"' blocks from the implementation files.

[libclang] Remove the 'extern "C"' blocks from the implementation files.

These are unnecessary, the declarations already carry the 'extern C' property, and if there is mismatch
between declaration and definition then we will get linker errors via libclang.exports.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@290025 91177308-0d34-0410-b5e6-96231b3b80d8