Overhaul CMake build. See history in origin/cmake-modern-history.

Fixes #870
Fixes #2643
Fixes #2821
Fixes #2852
Fixes #2942
Fixes #3658
Fixes #4009
Fixes #4284
Fixes #4476
Fixes #4581
Fixes #4893
Fixes #4895

.github/workflows/presubmit.yml

@@ -27,10 +27,11 @@ jobs:
       - uses: actions/checkout@v2
       - name: Install clang-tidy
         run: |
-          sudo apt-get install llvm-9 clang-tidy-9
+          sudo apt-get install llvm-9 clang-9 libclang-9-dev clang-tidy-9
+          sudo apt-get remove llvm-8 clang-8 libclang-8-dev clang-tidy-8
       - name: Build Compilation DB
         run: |
-          cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -DHALIDE_REQUIRE_LLVM_VERSION=90 -S . -B build
+          cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -DLLVM_DIR=/usr/lib/llvm-9/lib/cmake/llvm -S . -B build
           [ -a build/compile_commands.json ]
       - name: Run clang-tidy
         run: |

.gitignore

@@ -12,7 +12,6 @@ build-osx/*
 cmake_build*/*
 */build/*
 tmp/*
-doc/*
 include/*
 distrib/*
 testing/*
@@ -89,3 +88,5 @@ src/.tags_sorted_by_file
 
 /.vs
 /CMakeSettings.json
+/venv/
+/cmake-build-*/