Fixes #5072.
Also fixed some issues around LangVersion handling, which were found along the way.

Fixes #4924.

Fixes #4623

Fixes #4163.

Fixes # 4704.

...so that it doesn't wait indefinitely.

Channel lambda return type inference diagnostics to overload resolution in order to improve error reporting.

Fixes #4587.

to be the same as the analysis done by the native compiler, resolving
virtually all incompatibilities.
Fixes #4344

Adjust implementation of OverloadResolution.GetParameterCounts to account for a situation when params parameter is used through a named argument.

Fixes #4424.

Fixes #4620

Report a diagnostic if an analyzer reports a diagnostic with an invalid ID (has non-alphanumeric characters).

Stack spiller tried to save a temp and reuses spilled LHS to store the results of the whole expression.
However, the LHS is not always spilled into a temp. Readonly fields, for example, stay as they are.

As a result the optimization leads to modification of the readonly field which is not even verifiable.,

The change simply introduces a temp, which, if happens to be temp copy of a temp and thus unnecessary, is elided by the stack optimizer who is in a better position to reason about redundancy of locals.

Fixes #4628

This change does two actions to our public API files:

- Resets them to the versions from 1.0.0 RTM
- Splits them into the shipped and unshipped files

Move TestUtilities.csproj and TestUtilities.FX45.csproj to separate directories to accomodate current nuget limitations

The compiler wasn't properly handling the case of a conversion between
Nullable<int> and Nullable<IntPtr>.  The compiler specially handles
IntPtr conversions but wasn't properly lowering them in the
LocalRewriter.

I verified the output IL and expression trees are identical to the
native compiler for these cases.

This closes DevDiv 1210529

No need to use Visit to create a constant expression.

Many tests still need to be adjusted.

Native compiler exposes results of constant folding in the expression trees.
In particular, conversion from an enum literal to an underlying type is fairly consistently folded by the native compiler as it can be observed in the expression trees.

Roslyn, on the other hand, was not very consistent here.
Some cases like explicitly converted literals were folded - Ex:  IntTakingMethod((int)E1.a)
Other cases like implicit conversions to underlying type introduced in the processs of type promotion in binary expressions were not.

This fixes known differences in this behavior.

Fixes #4085
Fixes #3292

Fixes #2443

Fixes #4250

Fixes #3923

Once System.Runtime.Loader is referenced in the project.json files
a facade is correctly copied to the output directory, so
AssemblyLoadContext can be loaded even if the assembly it's located in
changes.

On CoreCLR we will always be unable to fetch the default encoding and
the 1252 encoding will be unavailable unless we reference
System.Text.Encoding.CodePages. However, we don't want to take a
dependency on this assembly on desktop 4.5 so we now use
AssemblyLoadContext to try to detect whether or not we are running on
CoreCLR and, if so, reflection load the necessary assemblies to get hold
of the necessary types.