Set QTC_CLANG_PLUGINS_LOAD env variable to get diagnostics
that come from libclang plugins.
This functionality works after the patch is applied to
clang repository and clang is properly built with it.

Task-number: QTCREATORBUG-15157
Change-Id: Iefeee4dd115f3f43ddc6ed79452a1135e653def6
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>