Enable LTO support for Chatterino builds (#4258)

This is disabled by default, and can be enabled by passing `-DCHATTERINO_LTO=On` to your cmake invocation.

.github/workflows/build.yml

@@ -12,6 +12,9 @@ concurrency:
   group: build-${{ github.ref }}
   cancel-in-progress: true
 
+env:
+  C2_ENABLE_LTO: ${{ github.ref == 'refs/heads/master' }}
+
 jobs:
   build:
     runs-on: ${{ matrix.os }}
@@ -20,17 +23,25 @@ jobs:
         os: [windows-latest, ubuntu-20.04, macos-latest]
         qt-version: [5.15.2, 5.12.12]
         pch: [true]
+        force-lto: [false]
         include:
           - os: ubuntu-20.04
             qt-version: 5.15.2
             pch: false
+            force-lto: true
       fail-fast: false
 
     steps:
+      - name: Force LTO
+        if: matrix.force-lto == true
+        run: |
+            echo "C2_ENABLE_LTO=ON" >> "$GITHUB_ENV"
+        shell: bash
+
       - name: Set environment variables for windows-latest
         if: matrix.os == 'windows-latest'
         run: |
-            echo "vs_version=2022" >> $GITHUB_ENV
+            echo "vs_version=2022" >> "$GITHUB_ENV"
         shell: bash
 
       - uses: actions/checkout@v3
@@ -80,7 +91,12 @@ jobs:
             mkdir build
             cd build
             conan install ..  -b missing
-            cmake -G"NMake Makefiles" -DCMAKE_BUILD_TYPE=Release -DUSE_CONAN=ON ..
+            cmake `
+                -G"NMake Makefiles" `
+                -DCMAKE_BUILD_TYPE=Release `
+                -DUSE_CONAN=ON `
+                -DCHATTERINO_LTO="$Env:C2_ENABLE_LTO" `
+                ..
             set cl=/MP
             nmake /S /NOLOGO
             windeployqt bin/chatterino.exe --release --no-compiler-runtime --no-translations --no-opengl-sw --dir Chatterino2/
@@ -137,8 +153,9 @@ jobs:
               -DPAJLADA_SETTINGS_USE_BOOST_FILESYSTEM=On \
               -DUSE_PRECOMPILED_HEADERS=${{ matrix.pch }} \
               -DCMAKE_EXPORT_COMPILE_COMMANDS=On \
+              -DCHATTERINO_LTO="$C2_ENABLE_LTO" \
               ..
-            make -j$(nproc)
+            make -j"$(nproc)"
         shell: bash
 
       - name: clang-tidy review
@@ -203,8 +220,9 @@ jobs:
                 -DCMAKE_OSX_DEPLOYMENT_TARGET=10.15 \
                 -DOPENSSL_ROOT_DIR=/usr/local/opt/openssl \
                 -DUSE_PRECOMPILED_HEADERS=${{ matrix.pch }} \
+                -DCHATTERINO_LTO="$C2_ENABLE_LTO" \
                 ..
-            make -j$(sysctl -n hw.logicalcpu)
+            make -j"$(sysctl -n hw.logicalcpu)"
         shell: bash
 
       - name: Package (MacOS)