IntelliSense should show conditional compilation symbols

[terrajobst]

Version Used: Visual Studio 2017 RC.3+26114.2.d15rel

Steps to Reproduce:

1. Create a new C# project
2. Add a conditional compilation symbol to your project (e.g. AWESOME)
3. Start typing #if and invoke IntellISense

Expected Behavior:

IntelliSense displays the list of conditional compilation symbols. Ideally, across all configurations but at least for the current project configuration (e.g. Debug|x86).

Actual Behavior:

It helpfully shows true and false. Hooray!

This is especially frustrating for projects that are cross-compiled (i.e. compile for multiple TFMs). The developer cannot easily discover the conditional compilation symbol for the TFM. I ended up running a diagnostic build to see what's passed to CSC.

[miloush]

A duplicate of #11499?

[terrajobst]

Thanks. Was searching, but was probably using the wrong keywords.

@srivatsn @davkean @Pilchie should we bump the priority? I expect cross compilation to become a lot more prominent, considering the improvements in our tooling. Not having IntelliSense for symbols makes this harder than it needs to be.