Update Travis CI configuration.

The Travis configuration:
1) Installs recent versions of clang and GCC.
2) Sets up the environment so that CMake picks up the installed
   compilers. Previously, the pre-installed clang compiler was used
   instead.
3) Requests a modern macOS image that has all the headers needed by GCC.

The CL also removes now-unnecessary old workarounds from the
Travis configuration.

PiperOrigin-RevId: 245832795

.travis.yml

@@ -2,28 +2,25 @@
 # http://about.travis-ci.org/docs/user/build-configuration/
 # This file can be validated on: http://lint.travis-ci.org/
 
-dist: xenial
 language: cpp
+dist: xenial
+osx_image: xcode10.2
 
 compiler:
-  - gcc
-  - clang
+- gcc
+- clang
 os:
-  - linux
-  - osx
+- linux
+- osx
 
 env:
-  - BUILD_TYPE=Debug CPU_LEVEL=AVX
-  - BUILD_TYPE=Debug CPU_LEVEL=AVX2
-  - BUILD_TYPE=RelWithDebInfo CPU_LEVEL=AVX
-  - BUILD_TYPE=RelWithDebInfo CPU_LEVEL=AVX2
+- BUILD_TYPE=Debug CPU_LEVEL=AVX
+- BUILD_TYPE=Debug CPU_LEVEL=AVX2
+- BUILD_TYPE=RelWithDebInfo CPU_LEVEL=AVX
+- BUILD_TYPE=RelWithDebInfo CPU_LEVEL=AVX2
 
 matrix:
   exclude:
-    # GCC fails on recent Travis OSX images.
-    # https://github.com/travis-ci/travis-ci/issues/9640
-    - compiler: gcc
-      os: osx
   # Travis OSX servers seem to run on pre-Haswell CPUs. Attempting to use AVX2
   # results in crashes.
   - env: BUILD_TYPE=Debug CPU_LEVEL=AVX2
@@ -33,15 +30,11 @@ matrix:
 
 addons:
   apt:
-    # List of whitelisted in travis packages for ubuntu-trusty can be found here:
-    #   https://github.com/travis-ci/apt-package-whitelist/blob/master/ubuntu-trusty
-    # List of whitelisted in travis apt-sources:
-    #   https://github.com/travis-ci/apt-source-whitelist/blob/master/ubuntu.json
     sources:
-    - llvm-toolchain-xenial-7
+    - llvm-toolchain-xenial-8
     - ubuntu-toolchain-r-test
     packages:
-    - clang-7
+    - clang-8
     - cmake
     - gcc-8
     - g++-8
@@ -50,18 +43,26 @@ addons:
     packages:
     - cmake
     - gcc@8
+    - llvm@8
     - ninja
-
-before_install:
-# The Travis VM image for Mac already has a link at /usr/local/include/c++,
-# causing Homebrew's gcc installation to error out. This was reported to
-# Homebrew maintainers at https://github.com/Homebrew/brew/issues/1742 and
-# removing the link emerged as a workaround.
-- if [ "$TRAVIS_OS_NAME" == "osx" ]; then rm -f /usr/local/include/c++ ; fi
+    update: true
 
 install:
-# /usr/bin/gcc is stuck to old versions on both Linux and OSX.
+# The following Homebrew packages aren't linked by default, and need to be
+# prepended to the path explicitly.
+- if [ "$TRAVIS_OS_NAME" == "osx" ]; then
+    export PATH="$(brew --prefix llvm)/bin:$PATH";
+  fi
+# /usr/bin/gcc points to an older compiler on both Linux and macOS.
 - if [ "$CXX" = "g++" ]; then export CXX="g++-8" CC="gcc-8"; fi
+# /usr/bin/clang points to an older compiler on both Linux and macOS.
+#
+# Homebrew's llvm package doesn't ship a versioned clang++ binary, so the values
+# below don't work on macOS. Fortunately, the path change above makes the
+# default values (clang and clang++) resolve to the correct compiler on macOS.
+- if [ "$TRAVIS_OS_NAME" == "linux" ]; then
+    if [ "$CXX" = "clang++" ]; then export CXX="clang++-8" CC="clang-8"; fi;
+  fi
 - echo ${CC}
 - echo ${CXX}
 - ${CXX} --version