Use clang to build release binary.

[PiotrSikora]

From the original PR:

Loading 1000 static listeners in a binary built with gcc takes ~51s,
but only ~1.5s (34x faster) in a binary built with clang.

The problem was mitigated by #4117, which reduced number of preallocated elements in the LcTrie, though the difference in performance still exists (due to unpacked LcNode).

cc @mattklein123 @htuch @alyssawilk @lizan

[PiotrSikora]

Note that we'd need to use gcc on CentOS (assuming we want to provide release binaries for it).

[mattklein123]

Note that we'd need to use gcc on CentOS (assuming we want to provide release binaries for it).

We don't provide binaries for CentOS today so I don't think we would start doing so unless someone in the community wants to properly resource CentOS support independently.

[uromahn]

@mattklein123 We are planning to introduce Envoy across our entire system and CentOS support is an absolute "must" for us. We are running our entire system on it and having everything re-certified with a different OS is just too expensive and time consuming.

I personally don't object the switch to clang as long as the current build for CentOS stays there.
I have experimented building a newer toolchain with clang6 for CentOS from source but the build is painfully slow and I wasn't getting much success so far (since I am not a C++ expert).