Introduce the notion of "Relocatable" precompiled headers, which are built

with a particular system root directory and can be used with a different
system root directory when the headers it depends on have been installed.
Relocatable precompiled headers rewrite the file names of the headers used
when generating the PCH file into the corresponding file names of the 
headers available when using the PCH file.

Addresses <rdar://problem/7001604>.

llvm-svn: 74885