Port "Fix BigInt literal error in ambient contexts when targeting < ES2020" (#1819)

Author: Andarist

internal/checker/grammarchecks.go

@@ -2145,7 +2145,8 @@ func (c *Checker) checkGrammarNumericLiteral(node *ast.NumericLiteral) {
 func (c *Checker) checkGrammarBigIntLiteral(node *ast.BigIntLiteral) bool {
 	literalType := ast.IsLiteralTypeNode(node.Parent) || ast.IsPrefixUnaryExpression(node.Parent) && ast.IsLiteralTypeNode(node.Parent.Parent)
 	if !literalType {
-		if c.languageVersion < core.ScriptTargetES2020 {
+		// Don't error on BigInt literals in ambient contexts
+		if node.Flags&ast.NodeFlagsAmbient == 0 && c.languageVersion < core.ScriptTargetES2020 {
 			if c.grammarErrorOnNode(node.AsNode(), diagnostics.BigInt_literals_are_not_available_when_targeting_lower_than_ES2020) {
 				return true
 			}

testdata/baselines/reference/submodule/compiler/bigintAmbientMinimal.errors.txt

@@ -1,24 +1,15 @@
-/ambient.d.ts(1,25): error TS2737: BigInt literals are not available when targeting lower than ES2020.
-/ambient.d.ts(3,19): error TS2737: BigInt literals are not available when targeting lower than ES2020.
-/main.ts(2,19): error TS2737: BigInt literals are not available when targeting lower than ES2020.
 /main.ts(5,17): error TS2737: BigInt literals are not available when targeting lower than ES2020.
 
 
-==== /ambient.d.ts (2 errors) ====
+==== /ambient.d.ts (0 errors) ====
     declare const fromDts = 789n;
-                            ~~~~
-!!! error TS2737: BigInt literals are not available when targeting lower than ES2020.
     declare namespace Lib {
         const value = 999n;
-                      ~~~~
-!!! error TS2737: BigInt literals are not available when targeting lower than ES2020.
     }
 
-==== /main.ts (2 errors) ====
+==== /main.ts (1 errors) ====
     // Minimal repro from issue
     declare const n = 123n;
-                      ~~~~
-!!! error TS2737: BigInt literals are not available when targeting lower than ES2020.
 
     // Non-ambient for comparison
     const regular = 456n;