Enable Travis CI for llvm_release_70 branch

For some reason find_package(LLVM) in travis environment (Ubuntu 16.04 Xenial
+ Cmake 3.12) find clang-tools-7 package instead of llvm-7-tools! So I added
-DCMAKE_PREFIX_PATH to work it around.

.travis.yml

@@ -13,12 +13,12 @@ git:
 branches:
   only:
     - master
+    - llvm_release_70
 
 addons:
   apt:
     sources:
-    - sourceline: 'deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial main'
-      key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
+    - llvm-toolchain-xenial-7
     - ubuntu-toolchain-r-test
     packages:
     - llvm-7-tools
@@ -69,8 +69,8 @@ script:
     if [ $BUILD_EXTERNAL == "0" ]; then
       mkdir llvm-spirv
       mv * llvm-spirv
-      git clone https://git.llvm.org/git/llvm.git/ --depth 1
-      mv llvm-spirv llvm/tools/llvm-spirv
+      git clone https://git.llvm.org/git/llvm.git/ --depth 1 -b release_70
+      mv llvm-spirv llvm/projects/llvm-spirv
     fi
   - mkdir build && cd build
   - |
@@ -81,6 +81,7 @@ script:
           -DBUILD_SHARED_LIBS=${SHARED_LIBS} \
           -DLLVM_BUILD_TOOLS=ON \
           -DLLVM_EXTERNAL_LIT="/usr/lib/llvm-7/build/utils/lit/lit.py" \
+          -DCMAKE_PREFIX_PATH="/usr/lib/llvm-7/" \
           -DLLVM_INCLUDE_TESTS=ON \
           -DCMAKE_INSTALL_PREFIX=../install/ \
           -DCMAKE_EXPORT_COMPILE_COMMANDS=ON \