Upgraded clang UBSan jobs to clang 10 in Travis CI.

.travis.yml

@@ -298,15 +298,16 @@ matrix:
               key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
 
     - os: linux
-      compiler: clang++-8
-      env: UBSAN=1 TOOLSET=clang COMPILER=clang++-8 CXXSTD=03,11,14,17,2a UBSAN_OPTIONS=print_stacktrace=1
+      compiler: clang++-10
+      env: UBSAN=1 TOOLSET=clang COMPILER=clang++-10 CXXSTD=03,11,14,17,2a UBSAN_OPTIONS=print_stacktrace=1
       addons:
         apt:
           packages:
-            - clang-8
+            - clang-10
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-xenial-8
+            - sourceline: 'deb https://apt.llvm.org/xenial/ llvm-toolchain-xenial-10 main'
+              key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
 
     - os: linux
       dist: trusty