Add missing mnemonics for grpc code generators.

Partial commit for third_party/*, see #17224.

Signed-off-by: Sunil Gowroji <sgowroji@google.com>

third_party/grpc/bazel/generate_cc.bzl

diff --git a/third_party/grpc/bazel/generate_cc.bzl b/third_party/grpc/bazel/generate_cc.bzl
index 89f25ea..63f46bf 100644
--- a/third_party/grpc/bazel/generate_cc.bzl
+++ b/third_party/grpc/bazel/generate_cc.bzl
@@ -137,6 +137,7 @@
         outputs = out_files,
         executable = ctx.executable.protoc,
         arguments = arguments,
+        mnemonic = "CppGrpcGenSource",
         use_default_shell_env = True,
     )
 

third_party/grpc/build_defs.bzl

diff --git a/third_party/grpc/build_defs.bzl b/third_party/grpc/build_defs.bzl
index 6129480..e685546 100644
--- a/third_party/grpc/build_defs.bzl
+++ b/third_party/grpc/build_defs.bzl
@@ -41,6 +41,7 @@
         inputs = srcs + includes,
         tools = [ctx.executable._java_plugin, ctx.executable._protoc],
         outputs = [srcdotjar],
+        mnemonic = "JavaGrpcGenSource",
         use_default_shell_env = True,
     )