build: add Go 1.15 to CircleCI build

.circleci/config.yml

@@ -387,6 +387,12 @@ jobs:
     steps:
       - test-linux:
           llvm: "10"
+  test-llvm10-go115:
+    docker:
+      - image: circleci/golang:1.15-buster
+    steps:
+      - test-linux:
+          llvm: "10"        
   assert-test-linux:
     docker:
       - image: circleci/golang:1.14-stretch
@@ -418,6 +424,7 @@ workflows:
       - test-llvm10-go112
       - test-llvm10-go113
       - test-llvm10-go114
+      - test-llvm10-go115
       - build-linux
       - build-macos
       - assert-test-linux