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

build_scripts/qt/compilers/clang.pri

@@ -1,5 +1,5 @@
 # comments are the value of CLANG_VERSION
-CLANG_VERSION = $$system("clang --version | grep 'clang version'")
+CLANG_VERSION = $$system("clang++ --version | grep 'clang version'")
 # clang version 6.0.0-1ubuntu2 (tags/RELEASE_600/final)
 CLANG_VERSION = $$split(CLANG_VERSION, ' ')
 # CLANG_VERSION is now a list, this is shown as spaces in QMAKE
@@ -12,29 +12,31 @@ CLANG_VERSION = $$member(CLANG_VERSION, 0)
 # 6
 
 greaterThan(CLANG_VERSION, 4) {
-    message('clang' version is above 4. Using regular clang.)
+    message('clang++' version is above 4. Using regular clang++.)
+    QMAKE_CXX = clang++
+    QMAKE_LINK = clang++
 }
 else {
-    message('clang' version is not above 4. Attempting to use highest specific version.)
+    message('clang++' version is not above 4. Attempting to use highest specific version.)
-    system("clang-5 --version") {
+    system("clang++-5 --version") {
-        QMAKE_CXX = clang-5
+        QMAKE_CXX = clang++-5
-        QMAKE_LINK = clang-5
+        QMAKE_LINK = clang++-5
-        message('clang-5' found.)
+        message('clang++-5' found.)
     }
-    system("clang-6 --version") {
+    system("clang++-6 --version") {
-        QMAKE_CXX = clang-6
+        QMAKE_CXX = clang++-6
-        QMAKE_LINK = clang-6
+        QMAKE_LINK = clang++-6
-        message('clang-6' found.)
+        message('clang++-6' found.)
     }
-    system("clang-7 --version") {
+    system("clang++-7 --version") {
-        QMAKE_CXX = clang-7
+        QMAKE_CXX = clang++-7
-        QMAKE_LINK = clang-7
+        QMAKE_LINK = clang++-7
-        message('clang-7' found.)
+        message('clang++-7' found.)
     }
-    system("clang-8 --version") {
+    system("clang++-8 --version") {
-        QMAKE_CXX = clang-8
+        QMAKE_CXX = clang++-8
-        QMAKE_LINK = clang-8
+        QMAKE_LINK = clang++-8
-        message('clang-8' found.)
+        message('clang++-8' found.)
     }
 }