[clang-tools] Fix cl::opt usage with Clang+PCH+dylib
Clang instantiates the vtable but not the method compare() of
OptionValueCopy<std::string> when using PCH. compare() is instantiated
as past of libLLVM.so, but due to -fvisibility-inlines-hidden, the
function is not visible when linking the tool. This might be a Clang
bug.
In any case, avoid this problem by disabling PCH.
