By default set CC to be clang for msan, asan and ubsan bazel build. (#843)

Previously typing "bazel build --config=msan" leads to a build failure
on environment that has gcc as default
and one has to type "CC=clang bazel build --config=msan".
With this change, only can do "bazel build --config=msan".

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
diff --git a/.bazelrc b/.bazelrc
index 55ffeb4..363e6d7 100644
--- a/.bazelrc
+++ b/.bazelrc
@@ -22,7 +22,8 @@
 #
 
 # Address sanitizer
-# CC=clang bazel build --config asan
+# bazel build --config asan
+build:asan --repo_env CC=clang
 build:asan --strip=never
 build:asan --copt -fsanitize=address
 build:asan --copt -DADDRESS_SANITIZER
@@ -32,7 +33,8 @@
 build:asan --linkopt -fsanitize=address
 
 # Memory sanitizer
-# CC=clang bazel build --config msan
+# bazel build --config msan
+build:msan --repo_env CC=clang
 build:msan --strip=never
 build:msan --copt -fsanitize=memory
 build:msan --copt -DADDRESS_SANITIZER
@@ -42,7 +44,8 @@
 build:msan --linkopt -fsanitize=memory
 
 # Undefined Behavior Sanitizer
-# CC=clang bazel build --config ubsan
+# bazel build --config ubsan
+build:ubsan --repo_env CC=clang
 build:ubsan --strip=never
 build:ubsan --copt -fsanitize=undefined
 build:ubsan --copt -g