clean up IncrementalFSharpBuild.frameworkTcImportsCache

src/fsharp/vs/IncrementalBuild.fs

@@ -1088,7 +1088,7 @@ module internal IncrementalFSharpBuild =
     /// Global service state
     type FrameworkImportsCacheKey = (*resolvedpath*)string list * string * (*ClrRoot*)string list* (*fsharpBinaries*)string
     let private frameworkTcImportsCache = AgedLookup<FrameworkImportsCacheKey,(TcGlobals * TcImports)>(8, areSame=(fun (x,y) -> x = y)) 
-
+    let ClearFrameworkTcImportsCache() = frameworkTcImportsCache.Clear()
     /// This function strips the "System" assemblies from the tcConfig and returns a age-cached TcImports for them.
     let GetFrameworkTcImports(tcConfig:TcConfig) =
         // Split into installed and not installed.

src/fsharp/vs/IncrementalBuild.fsi

@@ -50,7 +50,8 @@ module internal IncrementalFSharpBuild =
 
   /// Lookup the global static cache for building the FrameworkTcImports
   val GetFrameworkTcImports : TcConfig -> TcGlobals * TcImports * AssemblyResolution list * UnresolvedAssemblyReference list
-
+  val ClearFrameworkTcImportsCache: unit -> unit
+
   type PartialCheckResults = 
       { TcState : TcState 
         TcImports: TcImports 

src/fsharp/vs/service.fs

@@ -2999,6 +2999,7 @@ type FSharpChecker(projectCacheSize, keepAssemblyContents, keepAllBackgroundReso
         parseAndCheckFileInProjectCache.Clear()
         braceMatchCache.Clear()
         parseFileInProjectCache.Clear()
+        IncrementalFSharpBuild.ClearFrameworkTcImportsCache()
         for i in 0 .. 2 do 
             System.GC.Collect()
             System.GC.WaitForPendingFinalizers()