[compiler-rt] Build with C++17 explicitly

We've started using C++17 constructs in compiler-rt now (e.g.
string_view in ORC), but when using the bootstrapping build, we won't
inherit the C++ standard from LLVM, and compilation may fail if we
default to an older standard. Explicitly build compiler-rt with C++17 in
a standalone build, which matches what other subprojects (e.g. Clang and
LLD) do.