Disable LTO on Travis CI and change clang binary to clang++

Travis clang does not have the LLVMgold plugin installed.
Disabling LTO is easier than installing the plugin since the CI doesn't
create any artifacts anyway.

.travis.yml

@@ -31,6 +31,8 @@ before_install:
   - sudo apt install clang-format-8
   - clang-format-8 --version
   - python ./build_scripts/run-clang-format.py ./src -r --color always --clang-format-executable clang-format-8
+  - sed -i 's/QMAKE_CXXFLAGS += -flto//g' ./build_scripts/qt/compilers/clang-gcc-common-switches.pri # CI version of clang can't handle LTO
+  - sed -i 's/QMAKE_LFLAGS += -flto -fuse-ld=gold//g' ./build_scripts/qt/compilers/clang-gcc-common-switches.pri # CI version of clang can't handle LTO
 
 install:
   - sudo add-apt-repository ppa:beineri/opt-qt-5.12.7-bionic -y