[DebugInfo] Restore test case for long double constants.

Summary:
D27549 (partial fix for PR26619) emits a constant value in the debug
metadata for a floating-point static const that does not exceed 64
bits in size.  Whether or not a long double exceeds 64 bits in size
depends on the target.  Modify the test case so that it expects a
constant value for long double if and only if the long double is no
larger than 64 bits.

Reviewers: cfe-commits, probinson

Differential Revision: https://reviews.llvm.org/D27597

llvm-svn: 289686