Fix lexing of hexadecimal floating literals

Author: DougGregor

Sources/SwiftParser/Lexer.swift

@@ -1439,7 +1439,7 @@ extension Lexer.Cursor {
         return .integerLiteral
       }
 
-      self.advance(while: { $0.isDigit || $0 == Unicode.Scalar("_") })
+      self.advance(while: { $0.isHexDigit || $0 == Unicode.Scalar("_") })
 
       if !self.isAtEndOfFile, self.peek() != UInt8(ascii: "p") && self.peek() != UInt8(ascii: "P") {
         if !Unicode.Scalar(PtrOnDot!.peek(at: 1)).isDigit {

Tests/SwiftParserTest/LexerTests.swift

@@ -130,6 +130,7 @@ public class LexerTests: XCTestCase {
     0xff.0p2
     -0xff.0p2
     +0xff.0p2
+    0x1.921fb4p1
     """
     data.withUTF8 { buf in
       let lexemes = Lexer.lex(buf)
@@ -146,6 +147,7 @@ public class LexerTests: XCTestCase {
         lexeme(.floatingLiteral, "0xff.0p2"),
         lexeme(.prefixOperator, "\n+", leading: 1),
         lexeme(.floatingLiteral, "0xff.0p2"),
+        lexeme(.floatingLiteral, "\n0x1.921fb4p1", leading: 1),
         lexeme(.eof, ""),
       ])
     }