Special case Conversions and BadExpressions with missing syntax to ha…

Merge master to features/ioperation

Merge remote-tracking branch 'origin/master' into merges/master-to-features/ioperation-20170906-070016

Merge dev15.4.x to master

* Load DSRN from alternate path if set

* Fix error reporting

Add support for the CLI invoking Roslyn without CscToolPath

* dotnet/features/ioperation: (46 commits)
  Update broken tests.
  Remove Semantics.ConversionKind.
  updated field reference tests.
  Revert unrelated changes.
  Fix test after merge.
  Renamed tests appropriately.
  Added multiple declarator fixed statement test.
  Address PR feedback and refactor tests into smaller targeted unit tests
  Remove IsDeclaration: False from ILocalReferenceExpression and IFieldReferenceExpression. Adding back is tracked by #21798
  Updated fixed tests after merge.
  Add test to cover parenthesis simplification in a lambda expression
  Added fixed statement tests.
  Simplification should remove parentheses around the expression of an expression-bodied member
  Add a new boolean flag to IIsTypeExpression to represent VB TypeOf IsNot expression
  Added some test scenarios
  changed to assert checking non null
  Revert OperationKind.None filtering, added an exception for fixed statements in C#.
  Move the perf-correctness leg to 15.3
  Add ConfigureAwait.
  use methodSymbol.
  ...

Move IFixedStatement to internal

Don't write files in CompilerPackage if not needed

Merge pull request #20805 from dotnet/features/non-trailing

Disabling flakey VisualBasic.BasicEditAndContinue.MultiProjectDebuggingWhereNotAllModulesAreLoaded (#21926)

* disabling flakey test

* disabling all VB ENC tests

* dotnet/features/ioperation: (50 commits)
  Remove Semantics.ConversionKind.
  updated field reference tests.
  Revert unrelated changes.
  Fix test after merge.
  Renamed tests appropriately.
  Address PR feedback and refactor tests into smaller targeted unit tests
  Remove IsDeclaration: False from ILocalReferenceExpression and IFieldReferenceExpression. Adding back is tracked by #21798
  Remove todo comment, now tracked by #21797
  Add test to cover parenthesis simplification in a lambda expression
  Simplification should remove parentheses around the expression of an expression-bodied member
  Add a new boolean flag to IIsTypeExpression to represent VB TypeOf IsNot expression
  Added some test scenarios
  changed to assert checking non null
  Move the perf-correctness leg to 15.3
  Add ConfigureAwait.
  use methodSymbol.
  Fix instance reference test.
  Remove unusued variables.
  added couple interesting scenarios
  Force sign-extending cast when add/sub pointers and int32 in checked context
  ...

CompilerPackage was previously being lazy and would just delete and
rewrite files as needed when the package was loaded, even if the
machine would ultimately be left in the same state at the end.
For the old project system (which didn't monitor such file changes)
this wasn't bad, but with the new project system it now reloads projects
in response to changes which can be quite expensive.

Fixes dotnet/roslyn#21838.

Fixed Intellisense suggestion non-viable constructor call in delegating constructor

Merge latest bits from master branch into features/non-trailing

Add a regression test for https://github.com/dotnet/roslyn/issues/21766

Conflicts:
	src/Compilers/CSharp/Portable/Errors/MessageID.cs
	src/Compilers/VisualBasic/Portable/Parser/ParserFeature.vb

Move Roslyn to .NET Core 2.0 SDK

Bug fixes and testing for completion in XML documentation comments

Change Microbuild to no longer be forced to use the MSBuild that
Microbuild provides. Doing so means we rely on them to update their
machines when we need updates to MSBuild.

Instead just use our normal mechanisms for discovering MSBuild. This
doesn't depend on machine state.

https://github.com/dotnet/sdk/issues/1552