RFR: 8201647: Make initial clearing of CHeapBitMap optional

[Per Liden]

CHeapBitMap objects will by default clear the underlying bitmap during 
construction. In ZGC we don't want mark bitmaps to be cleared on 
construction, since we instead do this lazily during concurrent marking.

This patch adds a "bool clear = true" argument to CHeapBitMap's 
constructor to be able to opt-out of this behavior.

Bug: https://bugs.openjdk.java.net/browse/JDK-8201647
Webrev: http://cr.openjdk.java.net/~pliden/8201647/webrev.0

/Per