[PATCH] Work around a gcc x86-32 code generation bug

[Jim Stichnoth]

If LLVM is built with certain versions of gcc, as Release x86-32 binaries,
it may incorrectly transform 64-bit integer constants in the user's source
code.  Specifically, a constant like 0xfff0000000000001
that has the same bit representation as a signaling NaN may be transformed
to the quiet NaN representation.  Gcc 4.6.3 is known to have this bug, as
well as 4.7.

A simple workaround avoids triggering the gcc bug.

Manual searching for similar patterns in the Clang and LLVM code bases
found no other similar patterns, specifically a union containing a double
field and a 64-bit integer field, with a constructor that initializes the
double field.

Gcc bug report: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58416

On my x86-64 system, with gcc 4.6.3, the attached lit test triggers the bug
when LLVM is configured this way:
    CC=gcc CXX=g++ ../llvm/configure --enable-optimized
--with-extra-options=-m32 --with-extra-ld-options=-m32
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