Subzero: Improve class definition hygiene.

Delete zero-argument ctor where possible.

Delete default copy ctor and default assignment operator where possible (some were missed in the past).

(The above are not done to the cross tests because we aren't yet building them with C++11.)

Declare single-argument ctor as "explicit".

BUG= none
R=jfb@chromium.org

Review URL: https://codereview.chromium.org/952953002

src/IceELFObjectWriter.h

diff --git a/src/IceELFObjectWriter.h b/src/IceELFObjectWriter.h
index 6c44ed5..d27ee9c 100644
--- a/src/IceELFObjectWriter.h
+++ b/src/IceELFObjectWriter.h
@@ -48,6 +48,7 @@
 // file. Having both -fdata-sections and -ffunction-sections does allow
 // relaxing this requirement.
 class ELFObjectWriter {
+  ELFObjectWriter() = delete;
   ELFObjectWriter(const ELFObjectWriter &) = delete;
   ELFObjectWriter &operator=(const ELFObjectWriter &) = delete;