CMake: Add a build target for generating a source RPM

Summary:
'make srpm' or 'ninja srpm' will tar up the current source code and then
build a source RPM package.

By default it will use the llvm.spec file to generate the source RPM,
but you can specify your own custom spec file with the
LLVM_SRPM_USER_BINARY_SPECFILE option.  CMake will perform variable
substitution on your custom specfile, so you can reference CMake
variables in it.  For example:

Version:        @LLVM_RPM_SPEC_VERSION@

Note that everything in the source directory will be included in the
tarball so if you have a clang check out in tools/clang, then all
the clang source will end up in the tarball to.  It is recommended
to only use this build target with a clean source tree.

Reviewers: beanz

Reviewed By: beanz

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D30093

llvm-svn: 297007