Debug option for BuildTest and RunTest

As of now, only release version of CoreCLR Runtime NuGet package is
available. BuildTest and RunTest cannot be run with debug build option
yet.

Added code to give out warning message for developer when it is used.

Also documented this on GitHub wiki page here:
https://github.com/dotnet/llilc/wiki/Dev-Env-and-Test-Harness

test/LLILCEnv.ps1

@@ -58,6 +58,10 @@
     be downloaded and put in a default location. You can override
     the location by specifying environment variable LLILCTESTRESULT.
 
+    As of now, only release version of CoreCLR Runtime is available.
+    Debug version of CoreCLR Runtime support will be added when it
+    is available.
+
 .PARAMETER Arch
     Target Architecture
 
@@ -894,6 +898,12 @@ function Global:ExcludeTest([string]$Arch="x64", [string]$Build="Release")
 
 function Global:BuildTest([string]$Arch="x64", [string]$Build="Release")
 {
+  if ($Build -eq "Debug") {
+    Write-Warning ("Only Release CoreCLR Runtime available at this point.")
+    Write-Warning ("BuildTest cannot be performed with Debug CoreCLR Runtime yet.")
+    return
+  }
+
   $CoreCLRTestAssets = CoreCLRTestAssets
 
   pushd .
@@ -942,6 +952,12 @@ function Global:RunTest
   [string]$Dump="NoDump"
   )
 
+  if ($Build -eq "Debug") {
+    Write-Warning ("Only Release CoreCLR Runtime available at this point.")
+    Write-Warning ("RunTest cannot be performed with Debug CoreCLR Runtime yet.")
+    return $False
+  }
+
   $CoreCLRTestAssets = CoreCLRTestAssets
   $CoreCLRRuntime = CoreCLRRuntime
   $CoreCLRVersion = CoreCLRVersion