Cmake: Read the build.ninja file only once

The first getCmakeCXXFlags that reads the build.ninja file
stores the contents in the inout parameter cachedBuildNinja.
Further calls to getCMakeCXXFlags just use this cached value.

This prevents excessive locking of this file while constructing
the code model information.

Change-Id: I81e1431194bed6913c69abb2d954068c999a5c1b
Task-number: QTCREATORBUG-14639
Reviewed-by: Tom Tanner
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>