[DebugInfo] Common behavior for error types

Following D50807, and heading towards D50664, this intermediary change does the following:

1. Upgrade all custom Error types in llvm/trunk/lib/DebugInfo/ to use the new StringError behavior (D50807).
2. Implement std::is_error_code_enum and make_error_code() for DebugInfo error enumerations.
3. Rename GenericError -> PDBError (the file will be renamed in a subsequent commit)
4. Update custom error messages to follow the same formatting: (\w\s*)+\.
5. Keep generic "file not found" (ENOENT) errors as they are in PDB code. Previously, there used to be a custom enumeration for that purpose.
6. Remove a few extraneous LF in log() implementations. Printing LF is a responsability at a higher level, not at the error level.

Differential Revision: https://reviews.llvm.org/D51499

llvm-svn: 341228
diff --git a/llvm/lib/DebugInfo/PDB/Native/PDBFile.cpp b/llvm/lib/DebugInfo/PDB/Native/PDBFile.cpp
index c5ce3e0..a1f8786f 100644
--- a/llvm/lib/DebugInfo/PDB/Native/PDBFile.cpp
+++ b/llvm/lib/DebugInfo/PDB/Native/PDBFile.cpp
@@ -125,7 +125,7 @@
   if (auto EC = Reader.readObject(SB)) {
     consumeError(std::move(EC));
     return make_error<RawError>(raw_error_code::corrupt_file,
-                                "Does not contain superblock");
+                                "MSF superblock is missing");
   }
 
   if (auto EC = msf::validateSuperBlock(*SB))