Report network paths in paths substitutions

Module resolution refuses to read network paths, so a network `paths`
substitution only produced an unhelpful resolution failure. Report it
where it is written in the config instead.

Author: jakebailey

tsc/internal/compiler/program.go

@@ -21,6 +21,7 @@ import (
 	"github.com/microsoft/TypeScript/tsc/internal/locale"
 	"github.com/microsoft/TypeScript/tsc/internal/module"
 	"github.com/microsoft/TypeScript/tsc/internal/modulespecifiers"
+	"github.com/microsoft/TypeScript/tsc/internal/nativepath"
 	"github.com/microsoft/TypeScript/tsc/internal/outputpaths"
 	"github.com/microsoft/TypeScript/tsc/internal/packagejson"
 	"github.com/microsoft/TypeScript/tsc/internal/parser"
@@ -1181,6 +1182,8 @@ func (p *Program) verifyCompilerOptions() {
 			}
 			if !tspath.PathIsRelative(subst) && !tspath.PathIsAbsolute(subst) {
 				createDiagnosticForOptionPathKeyValue(key, i, diagnostics.Non_relative_paths_are_not_allowed_Did_you_forget_a_leading_Slash)
+			} else if nativepath.IsNetworkPath(tspath.GetNormalizedAbsolutePath(subst, options.GetPathsBasePath(p.GetCurrentDirectory()))) {
+				createDiagnosticForOptionPathKeyValue(key, i, diagnostics.Network_path_0_is_not_supported, subst)
 			}
 		}
 	}

tsc/testdata/baselines/reference/compiler/networkPathReferences.errors.txt

@@ -2,8 +2,24 @@
 /main.ts(2,22): error TS5116: Network path '//server/share/types.d.ts' is not supported.
 /main.ts(3,22): error TS5116: Network path '//?/UNC/server/share/types.d.ts' is not supported.
 /main.ts(4,22): error TS5116: Network path '/??/UNC/server/share/types.d.ts' is not supported.
+/tsconfig.json(4,24): error TS5116: Network path '//server/share/remote.d.ts' is not supported.
 
 
+==== /tsconfig.json (1 errors) ====
+    {
+        "compilerOptions": {
+            "paths": {
+                "remote": ["//server/share/remote.d.ts", "./share/types.d.ts"],
+                           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS5116: Network path '//server/share/remote.d.ts' is not supported.
+                "local": ["/??/../share/types.d.ts"]
+            }
+        }
+    }
+    
+==== /share/types.d.ts (0 errors) ====
+    export declare const x: number;
+    
 ==== /main.ts (4 errors) ====
     /// <reference path="//server/share/types.d.ts" />
                          ~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -20,7 +36,4 @@
     /// <reference path="./share/types.d.ts" />
     import { x } from "./share/types";
     x;
-    
-==== /share/types.d.ts (0 errors) ====
-    export declare const x: number;
 

tsc/testdata/tests/cases/compiler/networkPathReferences.ts

@@ -2,6 +2,16 @@
 // @module: preserve
 // @noTypesAndSymbols: true
 
+// @filename: /tsconfig.json
+{
+    "compilerOptions": {
+        "paths": {
+            "remote": ["//server/share/remote.d.ts", "./share/types.d.ts"],
+            "local": ["/??/../share/types.d.ts"]
+        }
+    }
+}
+
 // @filename: /share/types.d.ts
 export declare const x: number;