Implement basic support for importing source locations from one AST

into another AST, including their include history. Here's an example
error that involves a conflict merging a variable with different types
in two translation units (diagnosed in the third AST context into
which everything is merged).

/Volumes/Data/dgregor/Projects/llvm/tools/clang/test/ASTMerge/Inputs/var2.c:3:5:
error: external variable 'x2' declared with incompatible types in
different translation units ('int' vs. 'double')
int x2;
    ^
In file included from
/Volumes/Data/dgregor/Projects/llvm/tools/clang/test/ASTMerge/Inputs/var1.c:3:
/Volumes/Data/dgregor/Projects/llvm/tools/clang/test/ASTMerge/Inputs/var1.h:1:8:
note: declared here with type 'double'
double x2;
       ^

Although we maintain include history, we do not maintain macro
instantiation history across a merge. Instead, we map down to the
spelling location (for now!).

llvm-svn: 95732