[Coverage] Store compilation dir separately in coverage mapping

We currently always store absolute filenames in coverage mapping.  This
is problematic for several reasons. It poses a problem for distributed
compilation as source location might vary across machines.  We are also
duplicating the path prefix potentially wasting space.

This change modifies how we store filenames in coverage mapping. Rather
than absolute paths, it stores the compilation directory and file paths
as given to the compiler, either relative or absolute. Later when
reading the coverage mapping information, we recombine relative paths
with the working directory. This approach is similar to handling
ofDW_AT_comp_dir in DWARF.

Finally, we also provide a new option, -fprofile-compilation-dir akin
to -fdebug-compilation-dir which can be used to manually override the
compilation directory which is useful in distributed compilation cases.

Differential Revision: https://reviews.llvm.org/D95753

GitOrigin-RevId: 5fbd1a333aa1a0b70903d036b98ea56c51ae5224
diff --git a/include/profile/InstrProfData.inc b/include/profile/InstrProfData.inc
index 3ee7af7..9112c9f 100644
--- a/include/profile/InstrProfData.inc
+++ b/include/profile/InstrProfData.inc
@@ -649,7 +649,7 @@
 /* Indexed profile format version (start from 1). */
 /* Coverage mapping format version (start from 0). */
 /* Profile version is always of type uint64_t. Reserve the upper 8 bits in the
  * version for other variants of profile. We set the lowest bit of the upper 8