ci: upgrade Travis config to Ubuntu 18.04

Also in this commit:

 * Drop clang 3.8 (no easy way to install it on 18.04)
 * Add clang 10 (trunk)
 * Drop Xcode 8.3
 * Upgrade Xcode 10.2 to 10.3
 * Add Xcode 11

Change-Id: Ib845d45f7d86d1f8302786618b516b2534875e4e

.travis.yml

@@ -1,8 +1,7 @@
 language: cpp
-dist: xenial
+dist: bionic
 env:
   global:
-    - JOB_NAME=limited-build
     - WAF_JOBS=2
 
 matrix:
@@ -22,8 +21,6 @@ matrix:
 
     # Linux/clang
     # https://apt.llvm.org/
-    - os: linux
-      env: COMPILER=clang++-3.8
     - os: linux
       env: COMPILER=clang++-3.9
     - os: linux
@@ -32,20 +29,19 @@ matrix:
       env: COMPILER=clang++-5.0
     - os: linux
       env: COMPILER=clang++-6.0
-    # temporarily disable AddressSanitizer on clang-7 and later
+    # disable AddressSanitizer with clang 7 and 8
     # due to https://bugs.llvm.org/show_bug.cgi?id=40808
     - os: linux
       env: COMPILER=clang++-7 DISABLE_ASAN=yes
     - os: linux
       env: COMPILER=clang++-8 DISABLE_ASAN=yes
     - os: linux
-      env: COMPILER=clang++-9 DISABLE_ASAN=yes
+      env: COMPILER=clang++-9
+    - os: linux
+      env: COMPILER=clang++-10
 
     # macOS/clang
     # https://docs.travis-ci.com/user/reference/osx/#macos-version
-    - os: osx
-      osx_image: xcode8.3
-      env: OSX_VERSION=10.12
     - os: osx
       osx_image: xcode9.2
       env: OSX_VERSION=10.12
@@ -56,36 +52,38 @@ matrix:
       osx_image: xcode10.1
       env: OSX_VERSION=10.13
     - os: osx
-      osx_image: xcode10.2
+      osx_image: xcode10.3
+      env: OSX_VERSION=10.14
+    - os: osx
+      osx_image: xcode11
       env: OSX_VERSION=10.14
 
   allow_failures:
-    - env: COMPILER=clang++-9 DISABLE_ASAN=yes
+    - env: COMPILER=clang++-10
 
   fast_finish: true
 
-install: |
+before_install: |
     case ${COMPILER} in
-      g++-5)
+      g++-[567]|clang++-[3456].*)
         ;;
       g++-*)
         travis_retry sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
         travis_retry sudo apt-get -qq update
-        travis_retry sudo apt-get -qy install "${COMPILER}"
         ;;
       clang++-*)
-        CLANG_VERSION=${COMPILER/clang++}
-        if [[ ${CLANG_VERSION} != "-3."* ]]; then
-          travis_retry wget -nv -O - "https://apt.llvm.org/llvm-snapshot.gpg.key" | sudo apt-key add -
-          travis_retry sudo add-apt-repository -y "deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial${CLANG_VERSION/-9} main"
-        fi
+        LLVM_REPO=${COMPILER/clang++/llvm-toolchain-${TRAVIS_DIST}}
+        travis_retry wget -nv -O - "https://apt.llvm.org/llvm-snapshot.gpg.key" | sudo apt-key add -
+        travis_retry sudo add-apt-repository -y "deb http://apt.llvm.org/${TRAVIS_DIST}/ ${LLVM_REPO%-10} main"
         travis_retry sudo apt-get -qq update
-        travis_retry sudo apt-get -qy install "clang${CLANG_VERSION}"
         ;;
     esac
 
+install:
+  - if [[ -n ${COMPILER} ]]; then travis_retry sudo apt-get -qy install "${COMPILER/clang++/clang}"; fi
+
 before_script:
-  - if [[ ${TRAVIS_OS_NAME} == linux ]]; then export NODE_LABELS="Linux Ubuntu Ubuntu-16.04"; fi
+  - if [[ ${TRAVIS_OS_NAME} == linux ]]; then export NODE_LABELS="Linux Ubuntu Ubuntu-18.04"; fi
   - if [[ ${TRAVIS_OS_NAME} == osx ]]; then export NODE_LABELS="OSX OSX-${OSX_VERSION}"; fi
   - if [[ ${OSX_VERSION} == 10.12 ]]; then brew update; fi
   # workaround for https://github.com/Homebrew/homebrew-core/issues/26358