RTC_LOG(): Internally, pass logging severity as template argument
RTC_LOG(): Internally, pass logging severity as template argument

When the logging severity is statically known, passing it as a
template argument instead of as a function argument saves space at the
call site.

Because this is a constructor, it's not possible to pass template
arguments explicitly---they need to be deduced. So we pass a dummy
function argument whose type encodes the logging severity, and because
the dummy is an empty struct, the ABI generally specifies that this is
a no-op with no runtime cost.

In aggregate, this reduces the size of libjingle_peerconnection_so.so
by 4 kB.

Bug: webrtc:9185
Change-Id: I8118f39dc2aed3be34b2979a239fc0d3dffa969f
Reviewed-on: https://webrtc-review.googlesource.com/74582
Commit-Queue: Karl Wiberg <2c708eb0a58047bf403399f50b3da33ffd2dd444@webrtc.org>
Reviewed-by: Fredrik Solenberg <a86c06127c44de4932e34935a0755af178aabab6@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#23136}
TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc