[Linux] Don't keep the file descriptor for FileImageSource.

We don't really need it after calling `mmap()`, and not keeping it around
means it's easy to make sure it gets closed properly in all cases.

rdar://117590155

Author: al45tair

stdlib/public/Backtracing/FileImageSource.swift

@@ -24,7 +24,6 @@ enum FileImageSourceError: Error {
 }
 
 class FileImageSource: ImageSource {
-  private var _fd: Int32
   private var _mapping: UnsafeRawBufferPointer
 
   public var isMappedImage: Bool { return false }
@@ -38,15 +37,16 @@ class FileImageSource: ImageSource {
 
   public init(path: String) throws {
     _path = path
-    _fd = _swift_open(path, O_RDONLY, 0)
-    if _fd < 0 {
+    let fd = _swift_open(path, O_RDONLY, 0)
+    if fd < 0 {
       throw FileImageSourceError.posixError(_swift_get_errno())
     }
-    let size = lseek(_fd, 0, SEEK_END)
+    defer { close(fd) }
+    let size = lseek(fd, 0, SEEK_END)
     if size < 0 {
       throw FileImageSourceError.posixError(_swift_get_errno())
     }
-    let base = mmap(nil, Int(size), PROT_READ, MAP_FILE|MAP_PRIVATE, _fd, 0)
+    let base = mmap(nil, Int(size), PROT_READ, MAP_FILE|MAP_PRIVATE, fd, 0)
     if base == nil || base! == UnsafeRawPointer(bitPattern: -1)! {
       throw FileImageSourceError.posixError(_swift_get_errno())
     }
@@ -56,7 +56,6 @@ class FileImageSource: ImageSource {
   deinit {
     munmap(UnsafeMutableRawPointer(mutating: _mapping.baseAddress),
            _mapping.count)
-    close(_fd)
   }
 
   public func fetch<T>(from addr: Address,