ci: Travis: use gcc9 with gcov job [skip appveyor] (#10480)

* ci: Travis: use gcc-9 in gcov job

* ci: Travis: use CCACHE_CPP2=1

This is required to avoid warnings with newer gcc/clang.

Follow-up to: https://github.com/neovim/neovim/pull/10533

.travis.yml

@@ -46,8 +46,8 @@ env:
     - CCACHE_COMPRESS=1
     - CCACHE_SLOPPINESS=time_macros,file_macro
     - CCACHE_BASEDIR="$TRAVIS_BUILD_DIR"
-    # Default since 3.3, but Travis (Xenial) has 3.2.4; workaround (#10533).
-    - CCACHE_HASHDIR=1
+    # Default since 3.3, but Travis (Xenial) has 3.2.4; required with newer gcc/clang.
+    - CCACHE_CPP2=1
 
 anchors:
   envs: &common-job-env
@@ -55,6 +55,34 @@ anchors:
     # adds the target branch to the cache key.
     FOR_TRAVIS_CACHE=$TRAVIS_BRANCH
 
+addons:
+  apt:
+    packages: &common-apt-packages
+      - apport
+      - autoconf
+      - automake
+      - build-essential
+      - clang-4.0
+      - cmake
+      - cscope
+      - gcc-multilib
+      - gdb
+      - gperf
+      - language-pack-tr
+      - libc6-dev-i386
+      - libtool-bin
+      - locales
+      - ninja-build
+      - pkg-config
+      - unzip
+      - valgrind
+      - xclip
+  homebrew:
+    update: true
+    packages:
+      - ccache
+      - ninja
+
 jobs:
   include:
     - stage: baseline
@@ -106,14 +134,22 @@ jobs:
       env:
         - *common-job-env
 
-    - name: gcc-coverage
+    - name: gcc-coverage (gcc 9)
       os: linux
-      compiler: gcc
+      compiler: gcc-9
       env:
         - GCOV=gcov
         - CMAKE_FLAGS="$CMAKE_FLAGS -DUSE_GCOV=ON"
         - GCOV_ERROR_FILE="/tmp/libgcov-errors.log"
         - *common-job-env
+      addons:
+        apt:
+          sources:
+            - sourceline: 'ppa:ubuntu-toolchain-r/test'
+          packages:
+            - *common-apt-packages
+            - gcc-9
+
     - name: clang-tsan
       os: linux
       compiler: clang
@@ -130,34 +166,6 @@ before_script:  ci/before_script.sh
 script:         ci/script.sh
 before_cache:   ci/before_cache.sh
 
-addons:
-  apt:
-    packages:
-      - apport
-      - autoconf
-      - automake
-      - build-essential
-      - clang-4.0
-      - cmake
-      - cscope
-      - gcc-multilib
-      - gdb
-      - gperf
-      - language-pack-tr
-      - libc6-dev-i386
-      - libtool-bin
-      - locales
-      - ninja-build
-      - pkg-config
-      - unzip
-      - valgrind
-      - xclip
-  homebrew:
-    update: true
-    packages:
-      - ccache
-      - ninja
-
 branches:
   only:
     - master