[DebugInfo] Relax 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.  The regression test accompanying that fix assumes that
a long double exceeds 64 bits in size and hence does not get a
constant value in the debug metadata.  However, for some targets --
such as "--target=hexagon-unknown-elf" -- a long double does not
exceed 64 bits in size, and hence the test fails.

As a temporary fix, modify the regression test to no longer inspect
the debug metadata for a long double.

Reviewers: cfe-commits, probinson

Differential Revision: https://reviews.llvm.org/D27589

llvm-svn: 289103