[DebugInfo] Improvements to representation of enumeration types (PR36168)

This patch:

* fixes an incorrect sign-extension of unsigned values, when emitting
  debug info metadata for enumerators
* the enumerators metadata is created with a flag, which determines
  interpretation of the value bits (signed or unsigned)
* the enumerations metadata contains the underlying integer type and a
  flag, indicating whether this is a C++ "fixed enum"

Differential Revision: https://reviews.llvm.org/D42736

llvm-svn: 324490