RFR (L) JDK-8231844: Enhance type signature characters in classfile_constants.h and improve the JVM to use type signature characters more consistently

[Lois Foltan]

Please review this enhancement to consistently use the JVM_SIGNATURE_* 
constants for type signature processing.  This change is a precursor to 
JDK-8230199: consolidate signature parsing in Hotspot sources.

open webrev 
at:http://cr.openjdk.java.net/~lfoltan/bug_jdk8231844.0/webrev/ 
<http://cr.openjdk.java.net/~lfoltan/bug_jdk8231844.0/webrev/>
bug link: https://bugs.openjdk.java.net/browse/JDK-8231844
contributed-by: Lois Foltan, John Rose

Testing: hs-tier1-3, jdk-tier1-2 (all platforms).  hs-tier4-6 (linux only)

Thanks,
Lois