Clang UPC (and UPC2C) version string reflects the time of the build

I noticed that we are using this cmake command to acquire the clang UPC version (tools/clang/CMakeLists.txt):
```
EXECUTE_PROCESS(COMMAND "date" "+%Y%m%d" OUTPUT_VARIABLE CLANG_UPC_VERSION)
```
This puts the current date into the Makefile and clang-upc will output:
```
clang version 3.4  (UPC 2014XXXX)
```
The date is the date of the build, when cmake ran, rather then the date of last commit.

Also, 'clang --version' should not report UPC in its version string.  I think that the area in parentheses is reserved for OEM version (e.g. (Red Hat 4.7.2-8)).

The same is true for builds with configure/Makefile.