rust: Pass additional Clang C flags from toolchain

The set of Clang flags passed to bindgen from the toolchain was
incomplete. In addition to ToolchainClangCflags(), we also need to call
ClangCflags().

Bug: 184081465
Test: m libgrpc_bindgen
Change-Id: I0b62345fee2f55ba18b4a0c4d825f0447c99db06