Merge pull request #3050 from lambdageek/dev/monoerror-to_utf8

[runtime] Mark mono_string_to_utf8 external only.

diff --cc mono/metadata/appdomain.c
Simple merge

diff --cc mono/metadata/icall.c
Simple merge

diff --cc mono/metadata/object.c
Simple merge

diff --cc mono/metadata/reflection.c
Simple merge

diff --cc mono/mini/debugger-agent.c
Simple merge