Repro steps:

- create a new project
- add the LWRP package
- update the PostProcessing package
- Add a PostProcessing Layer to the MAin camera in the scene
- notice the screen will go black.
- You will get this shader error

Shader error in 'Hidden/PostProcessing/FinalPass': failed to open source file: '../StdLib.hlsl' at line 12 (on d3d11)

Compiling Fragment program with FXAA
Platform defines: UNITY_ENABLE_REFLECTION_BUFFERS UNITY_USE_DITHER_MASK_FOR_ALPHABLENDED_SHADOWS UNITY_PBS_USE_BRDF1 UNITY_SPECCUBE_BOX_PROJECTION UNITY_SPECCUBE_BLENDING UNITY_ENABLE_DETAIL_NORMALMAP SHADER_API_DESKTOP UNITY_COLORSPACE_GAMMA UNITY_LIGHT_PROBE_PROXY_VOLUME UNITY_LIGHTMAP_FULL_HDR

Reproduced in Unity 19.1.0b9 and 19.2.0a10
Cannot reproduce in 18.3, LWRP 4.10 and PP 2.1.5
Reproduced with PopstProcessing 2.1.5, 2.1.4 and LWRP 5.2.3 to 5.10.0
Cannot reproduce with PostProcessing 2.1.3

Once you switch to 2.1.3 and switch back to 2.1.4 or 2.1.5 the shader compile error is no longer reproducible
Once you reimport the PostProcessing package the shader compile error is no longer reproducible