[clang] Remove extra ';' in MultiplexExternalSemaSource.cpp (NFC)

/llvm-project/clang/lib/Sema/MultiplexExternalSemaSource.cpp:317:2:
error: extra ';' outside of a function is incompatible with C++98 [-Werror,-Wc++98-compat-extra-semi]
};
 ^
1 error generated.