Address NDEBUG-related linkage issues for Value::assertModuleIsMaterialized()

The IR/Value class had a linkage issue present when LLVM was built
as a library, and the LLVM library build time had different settings
for NDEBUG than the client of the LLVM library.  Clients could get
into a state where the LLVM lib expected
Value::assertModuleIsMaterialized() to be inline-defined in the header
but clients expected that method to be defined in the LLVM library.

See this llvm-commits thread for more details:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160201/329667.html

llvm-svn: 259695