Reproduction steps:
1. Open the project provided
2. Open the only shader available
3. Change "#define BUG 0" to "#define BUG 1"
4. Save the shader
5. Return to Unity editor
-Observe that the shader never compiles and Unity is frozen

Actual result: Shader never compiles and Unity editor freezes
Expected result: Error is thrown related to an extra curly bracket

Note: Having unexpected opening curly bracket produces an error as expected.
Note: After closing frozen Unity editor UnityShaderCompiler.exe process is still running and has to be closed manually
Note: Closing UnityShaderCompiler.exe makes the editor run again.

Reproduced on: 5.5.0b8, 5.4.2p1, 5.3.6p7, 5.2.5f1